The February Revolution started with people rioting over food prices in the capital Petrograd. When the soldiers wouldn't fire on the demonstrators, things got out of hand and it turned into a spontaneous mass uprising.
The result of the February Revolution was that Tsar Nicholas II was forced to abdicate. He was very unpopular. The First World War was going very badly for Russia and people blamed the Tsar.
After the Feburary Revolution, Russia got a new government called the Provisional Government. This consisted of liberals and moderate socialists who wanted to turn Russia into a parliamentary democracy.
The Provisional Government was opposed by the Bolsheviks, the hardline Marxists led by Lenin. Lenin had been in exile in Switzerland but was smuggled into Russia by the Germans in April (a very smart move on their part!).
The Bolsheviks were popular among the workers and the soldiers. This is mainly because Lenin promised to end the war immediately. His slogans were "Peace! Bread! Land!" and "All Power to the Soviets!". The Soviets were councils that were set up after the February Revolution to represent the workers and soldiers.
The other big problem facing the government was the same one that had faced the Tsar: the war. Unfortunately for the Provisional Government, Russia didn't have much of an army left. By now, all the soldiers wanted to do was go home.
The Bolsheviks took advantage of this discontent to launch a second "revolution" against the Provisional Government in November. Unlike the February Revolution, which had been a mass uprising, this one was carried out by a few thousand armed Bolshevik supporters, who seized key buildings in Petrograd.
After seizing power, Lenin...