There are libraries filled with answers to the first question. There is no quick, easy summation. It would necessarily be overly broad, and fallacious. The second question is answered with a literal translation of the Greek roots demos = the people, kratos = ruler. It is government by the people. However, pure democracy tends to turn into mob rule. Not only that, but the modern usage of the term conflates a pollitical system with an economic system, and people often say they want to spread democracy, when they really mean that they want to spread a free market economic system. In most current "democracies", the representative forms of government are actually far more similar to republics; however, they are distinguished from ancient republics by carefully wrought systems of checks and balances.
Your library should have the reference books necessary for your homework. Telling you the answers would be a grave disservice to you as, in all probably, part of the assignment is to help you learn how to research.