Every serious subject has a language of its own. Typography is no exception. The following diagram shows a few terms used to talk about letterforms. Many more appear in the glossary of typographic terms. These terms let you discuss type like an expert.
The serif, or cross-line at the end of a stroke, probably dates from early Rome. Father Edward Catich proposed in his seminal work, "The Origin of the Serif," that the serif is an artifact of brushing letters onto stone before cutting them. Serif types are useful in text because the serifs help distinguish individual letters and provide continuity for the reader's eye.
Serifs come in many styles. Compare the tapered serifs of Minion to the slab serifs of Rockwell:
Traditionally, x-height is the height of the lowercase letter x. It affects the feel of a typeface, how many characters fit on a line, and depending on how the type is set, how easily your text can be read. At very small point sizes, a font with a larger x-height is easier to read, everything else being equal. Compare the following examples of Garamond and Helvetica, both at a point size of 12: