What it is and where it comes from
The Quran forbids intoxicants (5:90). The Prophet said "every intoxicant is khamr, and every khamr is forbidden" (Muslim) and "what intoxicates in a large amount, a small amount of it is forbidden" (Abu Dawud, Tirmidhi). This covers wine, beer, cider, spirits and liqueurs.
Where you find it
Drinks, and food made with them: wine sauces, desserts with liqueur, rum cakes and pralines.
What to look for on the label
Look for wine, beer, rum, brandy, liqueur, kirsch, sherry, marsala and the word alcohol. For tiny amounts in flavourings, see alcohol in flavouring.
This is a guide, not a fatwa. Recipes change, so check the label, look for halal certification, and ask your local imam where scholars differ.