A style guide clarifies for writers how to handle unresolved grammar, spelling, and formatting problems, as well as general writing recommendations.
Here are some examples of issues that style guides address:
Grammar issue: Oxford comma use (as in above list)
Spelling issue: British or American spelling, such as cancelled vs. canceled
Formatting issue: Number of spaces after a period
General writing issue: Referring to yourself in first or third person (“I conducted this study” vs. “The researcher conducted this study”)
During the course of your graduate career, you will need to follow multiple style guides. First of all, your university may have a preferred style guide and your specific program may have its own particular guidelines. Furthermore, you absolutely must consult with adviser concerning his or her style preferences. Most likely, your university or program will prescribe a widely-used style guide such as APA, MLA, or Chicago. Learn more about style guides here.