A train on a dead-end track
In the days of World War I the tsarism was completely bankrupted from all points of view: economical, military, political, even religious. The infamous Raspoutin affair displayed the stupidity and lack of morality of the tsar and his family. In August 1915, following Raspoutin's advices, tsar Nicolai II took direct command over military operations. He was equally stupid in strategy as in everything else, and he became personally responsible for all military disasters caused by his commands. The tsar's court was also completely infiltrated by German agents, due to the fact, that the Romanov dinasty was of German origins and had many German relatives.
Therefore about 1916 everybody had enough of them. The Entente powers, England and France, wanted to have a stable Russian government, which would command its army with little more competence. Western intelligence started to encourage the pro-Western politicians of the liberal Constitutional Democratic party (the "kadets") to get rid of Nicolai II. The generals were as well anxious to dismiss their crowned commander-in-chief. The civilian state administration and political police and intelligence ("okhrana") was corrupted and incapable of anything but gaining more bribes for its officers. The ground was prepared for a revolution, as nobody no longer would defend the tsar's authority. In a dramatic night of 29/30 December 1916, prince Yussupov and his supporters murdered Raspoutin, preparing a kind of palace coup d'etat, aimed to preserve monarchy by assasinating the stupid monarch.
But it was too late for a mere palace coup. For Russian people the war meant nothing but death on the frontline or starvation behind it. The year 1916 saw many strikes, mutinies and rebellions, at first crushed in blood, but steadily getting more and more succesful.
On March 3rd, 1917 a strike occurs in great Putilov weaponry factory in Petrograd. Authorities reacted with a lock-out. Then a solidarity strike paralizes the whole city. On March 8th it's actually an uprising. Police is helpless. Soldiers refuse to fire at their fellow workers, and they join their ranks instead. On March 12 the whole city is in hands of the rebels, including local garrison and arsenal. The military trains, carrying loyal troops, are blocked by railway unions. The same happens with the royal train, on which Nicolai II tried to get back to his capital from his HQ in Mogilev. The tsar, imprisoned in a train on a dead-end track, signs his abdication, on favour of his younger brother (who was enough clever to refuse the crown).
On March 12th the first Workers and Soldiers Council (the Soviet) assembles in Petrograd. Its majority consists of moderate left parties (the socialrevolutionaries: SR's and the menscheviks). They decide to cooperate with liberal leaders of Duma (the parliament), who elected a Provisional Government. Thus a double structure of power arises: a normal, bourgeois government and workers, soldiers and peasants councils (soviets).
Among those two authorities, only the soviets were genuinely democratically elected. The Duma was elected long time ago, according to extremely oppressive ballot system, and its alienation was obvious even to its own members. Therefore the Provisional Government proclaimed to summon new election to a Constutitional Assembly, that would decide the future of Russia. The elections was endlessly postponed, as the new authority was afraid of radical mood of Russian people. They waited and waited for a better mood, until - kabong! - the October Revolution came (in November, of course).