When dealing with web pages, it's essentially "impossible" to be specific about what font-face is to be used in which circumstance. This is because as web page writers, we don't have any control* over what font-faces the client PC has available.
Tip #1 - if your design depends on using a specific font for some effect than reconsider using that effect.
Tip #2 - if your design must really use a specific font for some effect (see Tip #1 <g>) then consider embedding that effect into a graphic (PNG format, for example).
In the "old days" when I was learning graphics, the rule of thumb was even more limiting than FISH_A_HOLIC's suggestions: 2 faces. An they were typically a "sans-serif" font for the titles and headers and a "serif" font for the "content", or the other way around. I like to think in 35 years, that we've progressed a bit, but it's still important to keep that tied down.
Personally, I try to stick with one font face only. I set it in the common stylesheet for all my pages: "BODY { font-family: sans-serif; }" or "BODY (font-family: Verdana,"Trebuchet MS", Arial, sans-serif; }". The former simply defaults to whatever the user's browser has established as their default for the "sans-serif" font.
Then the variations, to keep the pages from being "too boring" come into play: different sizes and weights for different header levels (<H1>, <H2>, etc.). But again, not too much variation, otherwise the page ends up looking like a patchwork quilt.
*Downloadable Fonts: I made the bold statement that **essentially** we don't have any control over what fonts are installed on the client systems that attach to our site. There is a tool available that works in some browsers, from some servers that allow the site to ATTEMPT to upload a font to a client PC. The idea being that you can be ensured the client has your required font. But this is not universally supported, and even for systems that are capable of such an activity, the user might have font downloading turned OFF. So it has to be considered an unreliable accessory.
HTH
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