Although your computer has many fonts installed, when we specify a font in the coding of a web page, we need to ensure the use of "browser safe" fonts. Using a font that is not browser safe may look great on your computer, but if the viewer does not have that font installed, then the browser will revert to its default font. The default font is likely not the same size, thus the text may not fit within its designated space or be as readable as you expected. Also fonts may look different on Windows and Mac computers.
Basically, you should choose between:
Times New Roman (a serif font) and Arial (a sans-serif font).
Your site will then be coded to indicated a "font family" made up of fonts which look basically the same but are typical of different systems. Then if the first choice is not installed on a viewer's computer, one of the other choices in the "family" will be used instead of the browser's default. That will come closer to insuring the viewer sees what you see.
In those cases where we want to emphasize a something, like a title or heading, with one of the cool, but not-so-compatible fonts then we simply render that bit of text in an image.