Kerning: In typography, kerning (less commonly mortising) is the process of adjusting the spacing between characters in a proportional font, usually to achieve a visually pleasing result.
Leading: In typography, leading /ˈlɛdɪŋ/ refers to the distance between the baselines of successive lines of type. The term originated in the days of hand-typesetting, when thin strips of lead were inserted into the forms to increase the vertical distance between lines of type. The term is still used in modern page layout software such as QuarkXPress and Adobe InDesign.
Baseline: In European and West Asian typography and penmanship, the baseline is the line upon which most letters “sit” and below which descenders extend. In the example to the right, the letter ‘p’ has a descender; the other letters sit on the (red) baseline.
Tracking: Tracking is very similar to kerning in that it is the spacing between individual characters, but tracking is the space between groups of letters rather than individual letters.
Word Spacing: Over-tight word spacing causes words to appear to run into each other, making it more difficult for the reader to distinguish one word from the next. Conversely, word spacing that is too open – the more common occurrence – creates oversized blocks of white space between words, forcing the reader to read individual words rather than phrases or blocks of copy. This dramatically slows down the reading process, reducing reader comprehension and increasing the risk of distraction.