Whenever you write a paper, you should always cite your sources. Otherwise you could be guilty of plagiarism, which could result in failing the assignment, the class, or even expulsion from the college.
These sites aid you in creating citations by giving you fields to fill in, and then generating the citation for you. Make sure you always double-check your spelling, punctuation, and capitalization!
When you copy and paste a citation from a citation generator, you will need to be sure to double-space the lines and add a hanging indent (where all lines except the first one are indented 1/2 an inch). You will also need to check with the style manual for the citation style you are using for correct capitalization and proper format.
In order to make sure your citations are correct, you will need to be familiar with how each citation style should look. Here are some resources for the most popular citation styles.