There are 3 instance types defined. Compute, General & Memory. The major difference between these instance types is the CPU to Memory allocation as follows:
c1.* -- Compute Instance = 1 vCPU to 2GB of RAM g1.* -- General Instance = 1 vCPU to 4GB of RAM m1.* -- Memory Instance = 1 vCPU to 8GB of RAM
Each instance also has a specific amount of disk allocated.