NATO is an organization that includes countries in Europe and North America. The major reason for the formation of NATO was that the countries involved wanted a mutual defense alliance so that if one of them were attacked all of the other countries in NATO would come to their aide.
The Warsaw pact consisted mostly of the Soviet Union and its satellite countries. It was supposed to be another mutual defense alliance like the North Atlantic Treaty, and was created in response to it. The Soviet Union dominated the alliance. The Soviet Union kept military personnel in the other countries of the alliance and kept them running socialist governments. It was the first step in an attempt to strengthen the Soviet Union's hold on its satellite countries. This program was undertaken by Nikita Khrushchev and Nikolay Bulganin after they assumed power in early 1955. The United States and the Soviet Union were aganist each other in the cold war.