if you set "font-family: sans-serif" in your CSS, this is safe for every system, it will automatically select the nearest match to the standard sans serif font on the system it is being viewed from. This is because you are setting a font family, not a specific font
Other families that I know of from the top of my head are "mono" for monospaced fonts, and "serif" for the Times New Roman style fonts.
use fonts compatible with most platforms (windows in all its versions, macOS in all it's versions, linux or any other unix variant etc)
I usually use arial helvetica sans-serif verdana kind of fonts. For flash i usually use a flashpixel font that suits the subject. For headers, logo's etc you're free to go if you save the file as a bitmap. Never use too many fonts. 2 or in very rare occasions 3 per site is enough to me imho.
If you use a font not on the viewer's computer, they will simply render the page with their system default font, whatever that may be. Always use a standard font to avoid your pages being seriously altered by default fonts. Like many above, I recommend Arial. Arial is easy on the eyes and works well with professional sites. Especially good when a lot of information is being displayed.
I would suggest arial because it is widely used and professional. It i salso on every PC as a default font. The Helios family is also very professional, but if you make a template and sell just the template you'll have to include the fonts.....