Let me give you just two quotes.
1. "What is an analog computer?
Simply, an analog computer is a computing device that has two distinguishing characteristics:
1. Performs operations in a truly parallel manner. Meaning it can perform many calculations all at the same time.
2. And operates using continuous variables. Meaning it uses numbers that that change not in steps, but change in a smooth continuous manner.
By constrast, a digital computer can only perform sequential (one at a time) operations, and operates on discrete (noncontinuous) numbers. "
http://www.cowardstereoview.com/analog/
2. "An analog computer is a form of computer that uses the continuously changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved. In contrast, digital computers represent varying quantities symbolically, as their numerical values change. As an analog computer does not use discrete (exact) values, but rather continuous (approximate) values . . ."
http://en.wikipedia.org/wiki/Analog_computer