What It Scores
AB is a valid word in the tournament list this site uses. Tile values only, before any premium square: 4 in Scrabble, 5 in Words With Friends, 5 in Wordfeud. The letters break down as A1 B3.
Anagrams of AB (1)
Same letters, different order. Any of these plays the same tiles for the same score.
Longer Words Containing AB
AB appears inside each of these, which is what makes it worth knowing as a hook.
Next: the Word Unscrambler for any letters at all, and the two and three letter lists. Full entry and definition at Merriam-Webster.