X-height refers to the height of a lowercase letter in a typeface or the top of the midpoint of lowercase letter such as d, p, q and so on. Some typefaces have a tall x-height, some medium, and some have a short x-height. This drastically changes the look of the text depending on the x-height. A taller x-height can give a typeface a thinner appearance, and a shorter x-height can make a text look fatter or more round in appearance.
Take a look at the different x-heights in the letters above. The middle line is present to make it easier for comparison. Also, notice how all of the typefaces, even though they are the same point size, have variations in size appearances. This makes a typeface look smaller than it really is, so it is important to determine which typeface is best for your body copy before you lay out a design with it. Switching from Helvetica to Bodoni is going to have a drastic effect on not only the appearance of your text, but the flow as well. The same text, with the same point size will flow differently with a depending on the typeface. It is important to pay attention to the leading and the kerning.
Descenders act the same way. Different typefaces are going to have different descender lengths as well. 12 point Caslon Pro is going to have a slightly longer descender than Bodoni, which is a naturally rounder typeface. Above is a comparison of the different typefaces and their descenders. This is not only important to remember in body copy, but when using a typeface as a header or in a large format, it is important to think about the typeface, its width, x-height, and descender length. Is a rounder, thicker stemmed typeface suitable for your needs, or is a thinner look such as DIN Pro more appropriate? these are all things to think about when choosing your typefaces.