The Warsaw Pact was really very similar to NATO. In both cases, these were organizations that were created to try to bind together one of the two major Cold War blocs in an effort to project strength against the other bloc. The major similarity, then, is that both of these were organizations meant mainly to defend one side against the other.
A major difference was that the Warsaw Pact was also created as a way for the Soviet Union to maintain some amount of control over the rest of its bloc. The pact was created soon after Stalin died. At that point, there were moves by some Eastern European nations to pull away from the Soviet Union. The Warsaw Pact was meant to pull them closer. By contrast, NATO did not serve this purpose (as shown in how the US did not stop France from leaving NATO).