CISC : Complex instruction set computer.
The number of instructions in the opcode set is large. There are complex instructions available too, that can do the task of multiple simple opcodes. The processor size, speed and capabilities must be high for CISC. But is it is convenient for the programmer to write software programs. Also, the program size may be smaller. The processor could be costlier.
RISC: Reduced instruction set computer.
The number of instructions in the opcode set is less than in CISC. The instructions are simpler. The architecture of the processor is simpler and it may be smaller in size. The cost of the processor may be less. The program written using RISC opcodes will be longer. So there is optimization to be done using techniques. A program runs slower on RISC machines.