As I have understood the concept of modems, modems translate digital signal to analog ones and in reverse.
Well and then I've read the definition of a modem (technical book):
A modem converts digital direct current signals to digital alternating current signals and the other way around.
Well how does the word "analog" fit in there!? How can I get a picture of it? Can you explain a bit how "direct current" and "alternating current" fits in there?