Why the Q is a trap
There's one Q in the bag. It's worth 10 points, and nearly every English word that uses it also needs a U, a second specific tile you may never draw. Hold the Q too long and it jams your rack for turns. Get caught with it when someone goes out and it costs you 10 points at the end, and in most scoring variants your opponent gains what you lose. The whole problem goes away once you know the handful of words that play the Q without a U.
The essential four
QI (plural QIS): the life force in Chinese philosophy, and the most-played Q word in modern Scrabble. On a double-letter square the Q alone is worth 20. QAT (plural QATS): a shrub whose leaves are chewed as a stimulant, also spelled KAT and KHAT, both valid. QAID (plural QAIDS): a North African chief or local governor. QOPH (plural QOPHS): the nineteenth letter of the Hebrew alphabet. Learn these four and the Q stops being scary. Everything below is extra credit.
The longer list
QANAT, an underground irrigation channel from the ancient Middle East. QINDAR and QINTAR (and QINDARKA), units of Albanian currency, a gift when your rack is drowning in vowels and an N. FAQIR, a variant of fakir. SHEQEL (plural SHEQALIM), Israeli currency. TRANQ, slang for tranquilizer, and one of the few words that ends in Q. NIQAB, a face veil. QWERTY, the keyboard layout, valid in many newer lists. A few of these vary between dictionaries. This site checks against ENABLE; your app or club list may disagree on the newest ones, so test before you commit in a close game.
Q strategy beyond the word list
Knowing the words is half of it. The other half is knowing when not to use them. If the board offers no premium square for QI and your rack is otherwise healthy, dumping the Q for a flat 11 points wastes it. But holding it more than two or three turns waiting for the perfect spot is almost always worse. And don't forget the exchange. Trading the Q back to the bag costs a turn, but it frees your rack, and that's often worth more than a forced 12-point play. Simple rule: if holding the Q is costing you 10 or more points a turn in flexibility, let it go.
On defense, count it. There's only one. Once you've seen it played, every U in your rack drops in value. If it's late in the game and the Q hasn't shown up, don't leave yourself one tile from going out while it can still land on you.
Words With Friends notes
The core list works in WWF, where the Q is also worth 10. WWF's dictionary is a little looser and its board puts bonus squares in different places, so QI on a triple-letter square (30 for the Q) comes up even more often. And since WWF lets you test a word for free, trying a fringe word like QWERTY costs nothing.
Check any Q rack in the Scrabble finder or the WWF finder. More guides: every two-letter word, Wordle strategy, the history of anagrams.