In western typography, the baseline is the line upon which most letters 'sit' and below which descenders extend.
In the upper example shown to the right, the letter p has a descender and the other letters sit on the (red) baseline.
The x-height is the height of the body of lowercase letters in a font, excluding the ascenders and descenders. The x-height is typically, but not necessarily, the height of the lowercase letter x. The x-height can vary considerably from font to font at the same point size. Note that in many font designs the round shaped lowercase letters are extended a little bit above or below the x-height as part of their design, so as to achieve an optical effect of having the same x-height.
The ascender is the part of lowercase letters that extends above the x-height. In the upper example shown to the right, the letter h has an ascender.
Cap height is the height of a capital letter above the baseline. It specifically refers to the height of capital letters that are flat, such as H or I, as opposed to round letters such as O, or pointed letters like A, which may extend a little higher or lower to achieve an optical illusion of equal size.
Body height is the maximum height span throughout a font, between the top of the tallest capital letter or ascender to the bottom of the lowest descender. When you select some text the extents of the body height is highlighted either blue or purple, as in the lower example shown to the right.
Line spacing is the vertical distance of the baselines of consecutive lines in a paragraph, which may also be referred to as line height or baseline spacing.