There are many common symbols in mathematics that are difficult or awkward to create on a web page. Square roots, exponents, and integrals are some of the items which might trip you up. Some of these have fairly easy HTML solutions, while others do not.
Several solutions exist for the problems described above, but some are easier to create than others, and some may not be supported by your intended audience. To make the most of your math on the web, it pays to know several different ways to accomplish your task.
The easiest way to express your mathematical self on the Web is to use plain text. It's supported everywhere, and it transfers quickly over slow connections. The downside is that your math may look, um, slightly crude to your viewers. Nevertheless, there are lots of situations (email, newsgroups, Dr. Math questions...) when knowing how to use plain text to write math effectively will serve you well. Do not overlook plain text! See http://mathforum.org/typesetting/ for tips, tricks, and conventions.
The Graphing Calculator program (which comes free with most Macintosh computers) can be an excellent tool for creating many kinds of algebraic expressions. Its output typically looks quite neat. All of the following expressions were created with the Graphing Calculator:
Furthermore, Greg Robbins has recently released Graphing Calculator 2.0, which is superb at demonstrating mathematics.
For more advanced kinds of equations, you need a more powerful tool. Equation Editor (free with Microsoft Word, Works, and PowerPoint) can create many, many kinds of mathematical expressions. It was designed to format equations for Microsoft Word, but if you know the right tricks, you can use Equation Editor to create GIFs. Use the toolbars to create the math you want, then use your computer's built-in screen-snapshot capability to take a picture of the screen (copying and pasting generally results in bad news). Edit and crop as desired. The following expressions were created this way:
Design Science also makes a beefier commercial version of the program, called MathType. With it, you don't have to jump through as many hoops, since it can save equations directly as GIFs.
If you're making web pages to use with a known group of students (perhaps your own) and you can control which software they have on their computers, using a browser-side plug-in or other client-side software can be a real treat. Some of the stand-out products let you distribute interactive Maple or Mathematica notebooks over the web.
The down side to using a plug-in is that you'll limit your nice web gems to a much smaller audience. Most casual browsers won't take the time to download a plug-in just to view your web pages, because they don't know whether what you have will even be worth it. Also, many people believe that unfamiliar plug-ins have a tendency to crash their computers - and they're often right.