Each typeface is made up of different elements that distinguish it from other typefaces. Unless you are specially going to go into type design and looking to create brand new fonts, web designers don't generally need to know the specifics of typeface anatomy. If you're interested in learning more about these building blocks of typefaces and letterforms, there is a great article on typeface anatomy on the About.com desktop publishing site. At a basic level, the elements of typeface anatomy that you should be aware of are:

Cap and x-height: This is the height of the capital letters in the typeface and the height of the letter x. It tells you how tall the largest letters will be, as well as how big most lowercase letters will be. Fonts are sized based on these two characteristics.

Decenders and ascenders: These are the portions of letters that go below and above the x-height line. These typically refer to lowercase letters. For instance, the letter "b" has an ascender (the piece that sticks "up" from the letter) while the letter "p" has a descender (the parts that dips "down" from the letter).