The detailed rules of citation styles can seem arbitrary. It is helpful to remember that the point of citation is to become part of the scholarly conversation: this cycle of creation, dissemination, evaluation, critique, and exchange of ideas is an essential aspect of scholarship. Citation helps your reader to locate the materials you have used, and allows you to properly credit other researchers' ideas in your work.
You may be asked to use a specific citation style. Some disciplines have their own style. Part of developing mastery in your discipline will be becoming versed in the style you are expected to use.
Failure to acknowledge sources of information properly may constitute plagiarism. For an explicit definition of plagiarism, see the Boston University Academic Conduct Code.