Mahayana is longer and easier path towards enlightenment and Vajrayana is quicker and more harsh and hardworking path towards the achievement of enlightenment.
Different schools of Buddhism. Theravada means 'way of the elders', and is evolved from the original form of Buddhism. Mahayana means 'the greater vehicle', and has an unknown origin.
Theravada is a much older school of Buddhism, and more strict. It is less practiced than Mahayana. There are detailed articles about each of these branches of Buddhism on wikipedia.