A Style Guide helps you answer questions about many things, such as how to format both your paper itself AND the citations, both in text and the list of sources used you will include at the end of your paper.

MLA stands for Modern Language Association. The MLA Style Guide is most commonly used for language and Humanities scholarship. It is often the one used by high school students regardless of the discipline they are writing in. However, there are other styles used in different disciplines.

Yes. If you are writing in the sciences, the most commonly used style is APA (American Psychological Association), History often uses Chicago/Turabian. The different styles emphasize different aspects of documentation that are important in their specific disciplines.

Scholarly writing is a conversation. It is important that readers and scholars be able to find the sources another writer uses in order to continue furthering the conversation about their research. A "comprehensible, verifiable means of referring" (MLA 8th edition 5) to others' work is the only way the conversation can work as a means of growing knowledge. By using a standard style, scholars are able to refer to the work of their colleagues and predecessors in the field. Later scholars can find and read the work based on the references. Your references will include the version of of your source--which is important in the Internet Age--as well as the date you accessed something. Both of these are important to help readers find the sources you used.