What?
Communal farms, Kolkhoz, were set up all over the countryside, for a variety of different reasons. Stalin planned to merge all the small farms onto larger 'collective' farms. These new larger farms (Kolkhoz's) would pool labour and resources, and therefore would operate more effectively and efficiency. In addition, state provided tractors and fertilisers would modernise production, again making the operations more efficient.
Why?
Stalin had a variety of different reasons for collectivising the county, economic, ideological and political. Collectivisation was the communists long term aim for agriculture. However, in early 1929, few communists could have predicted the speed with which the system was to be introduced.
Economic factors
1926 saw record harvests for Russia, however 1927, 1928 and 1929 were all poor. The decrease in production meant that prices rose and there was not a 'surplus' to be exported in exchange form industrial materials, e.g. machinery and expertise.
Ideological
communists had done little to change Russian agriculture. Still using traditional techniques, and had distinct lack of revolutionary sprit – farmed for themselves rather than for the good, of 'mother Russia' This lead many communists to believe that collectivisation was essencial for the capitalist peasants to embrace socialism.
Political
Stalin was motivated to introduce collectivisation from his struggle with Bukharin and the right of the party, who were in favour of the NEP. Collectivisation was far more popular than the right wing alturnative of importing grain. As that would slow the pace of industrialiation.
When?
1927 - Stalin declared that the way forwards was collectivisation was the way forwards in the countryside.
1929 – collectivisation made compulsary
1939 – 99& of land had been collectivised and 90% of peasants lived on one of 250,000 Kolkhov's. The government took 90% of produce and the rest was left for people to live off.