Times New Roman, Comic Sans, and Calibri are different types of fonts. Some other common fonts are Arial, Georgia, and Verdana. Depending on if you are writing something formal or something fun, you would pick a certain font.
Usually if you are writing something formal, you would want to pick a font like Times New Roman. If you are writing something fun, you may want to use a font like Comic Sans.
Font variations are bold, italics, etc. Font sizes are the different sizes of the text. All of these different fonts, font sizes, and font variations can be found in Microsoft Word.