What's Transsib is? What's one have to understand under this word?

Transsib, Trans-Siberian way (modern names) or Great Siberian Way (historical name) is an excellent equipped rail-track through the all continent connecting European Russia, its largest industrial areas and the capital Moscow with its middle (Siberia) and eastern (Far East) areas. This  railway consolidate Russia, which is a country laying through 10 time zones, in unified economic organism but more main in unified war-strategy territory. If it wasn't be built in due time Russia with very much probability wasn't likely kept Far East and Pacific ocean coast for itself - as it couldn't kept Alaska not connecting with Russian Empire by stable means of communication. Also Transsib is the way gave a stimulus to development of eastern areas and involved them in economic life of another part of the huge country.
Some people think that it's necessary to explain the term "Transsib" as the way connecting Ural and Far East and literally passing "via" Siberia (Trans-Siberian). But it's at variance with state of things and doesn't reflect true meaning of the railway. But what's about the name? The name was given by English nicknamed the way not as "Great Siberian Way" as literally translation from Russian was to be but as "Trans-Siberian Railway" which later instilled and took root in speech.
And now "Transsib" as geo-political conception has a meaning as way connecting Center and Pacific Ocean, Moscow and Vladivostok, and a little broader as way connecting ports of West and the capital of Russia, and also exits to Europe (Moscow, St-Petersburg, Brest, Kaliningrad) with ports of East and exits to Asia (Vladivostok, Nakhodka, Vanino, Zabaikalsk); but not a local way connecting Ural with Far East.
Limited interpretation of the term "Transsib" supposes that we tell about the main passenger way Moscow - Yaroslavl - Yekaterinburg - Omsk - Irkutsk - Chita - Vladivostok, the precise way of it is given below.

The length of Transsib.

Actual length of Trans-Siberian railway by the main passenger way (from Moscow to Vladivostok) is 9288,2 km and by this indicator it's the longest railway on the planet running across by land almost all Eurasia. Tariff length (cost of tickets is calculated by it) is a little more - 9298 km and doesn't coincide with concrete one. There some cargo around lines at various sections. Transsib track wide is 1520 mm.
The length of Great Siberian Way before the First World War from St-Petersburg to Vladivostok by northern passenger way (via Vologda - Perm - Yeakaterinburg - Omsk - Chita - Harbin) was 8913 versts or 9508 km.
Transsib passes through territories of two parts of the world: Europe (0 - 1777 km) and Asia (1778 - 9289 km). 19.1% of Transsib length is in Europe and 80.9% of the length is in Asia.

The beginning and the end of the way.

Today the first point of Transsib is Yaroslavskiy terminus in Moscow, and the end point is Vladivostok terminal.
But it wasn't be so always: about up to the midle of the twenties the gateway to Siberia and to Far East was Kazanskiy (then Ryazanskiy) terminus and in very initial period of Transsib existence (the beginning of 20 century) it was Kursko-Nizhegorodskiy (now Kurskiy) terminus in Moscow. Also it's necassary to mention the before the revolution in 1917 the beginning point of Great Siberian Way was Moscovskiy terminus in St-Petersburg, the capital of Russian Empire. Also Vladivostok not always was the end point: brief time from the end of the ninities of the 19 century up to decisive ground battles of Russian-Japanese war in 1904-05 contemporaries counted that the completion of Great Siberian Way was marine fortress and city Port-Arthur situated at the coastline of East Chinese sea on rented Chinese Lyaodun peninsula.
About geographical borders of Transsib (the extreme points at west, east, north and south) you can read here.

Building: main landmarks.

The beginning of building: May 19 (31), 1891 in area close by Vladivostok (Kuperovskaya fold), czesarevich Nikolai Alexandrovich, future emperor Nikolai II attended at the laying.
Actual beginning of construction occured a little earlier in the beginning of March 1891 when construction of section Miass - Chelyabinsk began.
Coupling of rails at all length of Great Siberian Way took place on October 21 (November 3), 1901 when builders of Chinese Eastern line who lead rail-track from west and east met together. But regular rail traffic at all length of the railway wasn't be at that time.
Regular communication between capital of the empire St-Petersburg and Pacific ocean Russian ports Vladivostok and Dalni by railway was established in July 1903 when Chinese Eastern line passing through Manchuria was accepted to regular ("right") operation. The date July 1 (14), 1903 also marked by itself putting Great Siberian Way into service at all its length although there was a break in rail-track because it was necessary to ferry trains over Baikal by special train ferry.
Continous rail-track between St-Petersburg and Vladivostok appeared after beginning of working traffic by Circum-Baikal railway on September 18 (October 1), 1904 and one year later on October 16 (29), 1905 Circum-Baikal railway as a section of Great Siberian Way was accepted to regular operation; and regular passenger trains first in history obtained a possibiliti ti pass only by rails, without using of any ferries from Atlantic ocean shore (from Westen Europe) to Pacific ocean shore (to Vladivostok).
The end of buildin on Russian Empire territory was on October 5 (18), 1916 with starting of bridge over Amur near Khabarovsk and beginning of rail-traffic over this bridge.
The value of Transsib construction from 1891 to 1913 was 1.455.413 thousand roubles, about value of building of Great Siberian Way specific sections you can read here in detail.

Modern way of Transsib.

From 1956 Transsib route is next: Moscow-Yaroslavskaya - Yaroslavl-Main - Danilov - Bui - Shar'ya - Kirov - Balezino - Perm-2 - Yekaterinburg-Passenger - Tyumen - Nazyvaevskaya - Omsk-Passenger - Barabinsk - Novosibirsk-Main - Mariinsk - Achinsk-1 - Krasnoyarsk - Ilanskaya - Taishet - Nizhneudinsk - Zima - Irkutsk-Passenger - Slyudyanka-1 - Ulan-Ude - Petrovskiy Zavod - Chita-2 - Karymskaya - Chernyshevsk-Zabaikalskiy - Mogocha - Skovorodino - Belogorsk - Arkhara - Khabarovsk-1 - Vyazemskaya - Ruzhino - Ussuriisk - Vladivostok. This is the main passenger way of Transsib. Finally it formed to the beginning of the thirties when normal operation of shorter Chinese Eastern line became impossible through war-political causes and South Uralian way was too overloaded owning to USSR industrialization began to that moment.
Until 1949 in Baikal area Transsib main way passed by Circum-Baikal railway: via Irkutsk - along Angara river - Baikal station - along Baikal shore - to Slyudyanka station; in 1949-56 two routes functioned: old one along Baikal shore and new mountain pass one. Moreover passing route first was built in 1-track version (1941-1948) but it became 2-tracks and the main one to 1957.
From June 10, 2001, after starting new summer shedule, almost all long-distance Trans-Siberian trains were went by new route via Vladimir - Nizhniy Novgorod with way out to "classical way" in Kotelnich. This way allows to pass trains with higher route speed. Moreover Transsib kilometrage as before is passing via Yaroslavl - Shar'ya.

Historical way of Transsib.

Up to the revolution in 1917 and some time later (until the end of the twenties of the 20 century) main way of Great Siberian Way passed:
From Moscow, from 1904: via Ryazan - Ryazhsk - Penza - Syzran - Samara - Ufa - Chelyabinsk - Kurgan - Petropavlovsk - Omsk - Krasnoyarsk - Irkutsk - Baikal - Mysovaya - Verkhneudinsk - Chita - Manchuria - Kharbin - Grodekovo - to Vladivostok. But it was short period (1897 - 1903) when from Moscow Siberian trains passed via Tula - Uzlovaya - Ryazhsk and after by the route given above.
From St-Petersburg, from 1906: via Cherepovets - Vologda - Bui - Shar'ya - Vyatka - Perm - Nizhniy Tagil - Yekaterinburg - Kyshtym - Chelyabinsk - Omsk, further similary previous one to Vladivostok. In 1909 this way was squared up - from Perm to Yekaterinburg shorter railway via Kungur was built and from October 1913 trains from the empire capital passed by some more shorter way - from Yekaterinburg via Tyumen to Omsk.
Up to April 1905 also South Manchurian line from Harbin via Changchun to port Dalni and marine fortress Port-Arthur belonged to Russia.

Cities, rivers and lakes on the way.

Transsib pass through territories of 12 regions, 5 territories, 2 republics, 1 autonomous region and 1  district included in a region of Russian Federation and 87 town are situated at it. [more detailed about it]
Transsib crosses 16 large rivers: Volga, Vyatka, Kama, Tobol, Irtysh, Ob, Tom, Chulym, Yenisei, Oka, Selenga, Zeya, Bureya, Amur, Khor, Ussuri; over 207 km it passes along Baikal lake and over 39 km by the shore of Amursky Bay of Japanese Sea.

Railway electrification.

By state on January 1, 2003 on Transsib 9289 are electrified, or 100% full length of the railway, including:
at alternating current (~25 kV) - 6412 km (Danilov - Balezino 836 km, Mariinsk - Vladivostok 5576 km),
at direct current (=3 kV) - 2877 km (Moscow-Yaroslavskaya - Danilov 356 km, Balezino - Mariinsk 2521 km). [more detailed about it]
Today no one non-electified section left - on December 25, 2002 the last section Guberovo - Ruzhino - Sviyagino was put undet voltage and electric locomotives passed by it.
The greatest rates of electirication were achieved in 1960 when 947 km of electified sections were put into service. In 1961 to the beginning of work of XXII Congress of the Communist Party of the Soviet Union completely electified cargo way from Moscow to Baikal via Ryazan - Ruzaevka - Ufa - Chelyabinsk was put into service, in 1994 way from Moscow to Khabarovsk was electrified.