Most business-oriented users never need to touch their fonts—nor should they. That pleasure is usually reserved for graphic designers, editors, and those requiring company-specific fonts. There are two ways to go about editing existing fonts and installing new ones: manually and with a font manager.
Generally, there are three types of fonts you will run into:
OpenType (OT) fonts are the newest, although they have been around since 1997. The best thing about OT fonts is that they are cross-platform between Windows and Macintosh computers. In addition, depending on the font, they can contain thousands of characters or glyphs; which is great for custom typesetting or accessing foreign characters.
TypeType (TT) fonts are very similar to OpenType fonts, except they are not cross-platform...well, sort of. Windows TT fonts can be used and installed on a Mac. However, Windows cannot use Mac TT fonts.
Type 1 (T1) fonts, often called PostScript fonts, were the mainstay of the publishing business and are still commonly used by many graphic designers (mostly on Macs). They have limited characters (256) and are not cross platform. The same font could be purchased for both platforms (doubling the cost over OT), but even then there is no guarantee that the text would not reflow. Luckily, Type 1 fonts are getting rare to find today since most foundries (font design companies) have switched to OpenType fonts.
To install or remove fonts from Windows manually, one must use the Fonts control panel. (I'm mostly talking about Windows 7 here.) Not all fonts can be removed, as some are required by Windows. Fortunately, Windows will not let you remove required fonts. To remove a font manually, simply right-click on it and select Delete; however, I strongly suggest making a copy of the font in another folder before deleting it. To add fonts, simply drag or copy the font in. You may need to be an admin user to edit fonts.
Mac users have a slightly more complicated situation when dealing with fonts: there are at least three places they can be located. They are in always in a Library/Fonts folder—but the Library folders are located on the root directory; the System folder; and, the user's home folder. Generally, I suggest installing fonts manually in the ~user/Library/Fonts folder to protect the operating system from corrupt fonts. If your ~user/Library folder is not visible, hold down the Option key and go up to the Go menu in the Finder. You will need to be an admin user to edit the System and root Library fonts, but not the ~user Library fonts.
The best option is to replace any T1 and TT fonts with OpenType versions. Unfortunately, most font companies don't offer upgrades from old versions, so the fonts must be purchased anew.
As I mentioned, Mac users can use Windows TT fonts (and any Windows OT fonts, which are by definition cross-platform). This is important when sharing files with WIndows users. Mac users have what I call "Microsoft fonts" (Arial, Times New Roman, Windings, etc.) when they have installed Microsoft Office for the Mac. Unfortunately, the Mac version of those fonts is not the same as the Windows version of fonts. I simply remove the Mac versions of the Microsoft fonts and replace them with Windows versions.
Mac users should avoid using Mac-only fonts, such as Helvetica and Helvetica Neue dFont (a Mac-only type of TT fonts). Also, Mac InDesign users could install Windows T1 fonts into the InDesign Fonts application folder, but this should only be used as a last resort.
Watch out for the licensing restrictions from "boutique" foundries (boutique is the term given for small font design companies; a foundry is the name for companies that design fonts). Many legally limit your usage as to what you can do with documents created with the fonts. This might include using them in websites, in PDFs that you publicly post or distribute, or even files you send to an printing company. Ideally, I would suggest not using those type of fonts (and be sure to send the company an email explaining why you are not using their products). However, sometimes you have to. In that case, it is usually a matter of paying an extra fee (which really means the license restriction is simply a way to make more money). BUT, if the foundry finds you have been using the font improperly, even out of ignorance, you could get a letter from their lawyers along with a nice copyright-infringment bill attached.