What it is and where it comes from
Milk is halal. What decides is the rennet that sets the cheese, and sometimes added enzymes (lipase) or alcohol. Most supermarket cheese is now made with microbial rennet or chymosin from fermentation, but many traditional and protected cheeses still use calf rennet. A few cheeses are washed or soaked in wine or beer.
Where you find it
Cheese itself, and cheese in pizza, sauces, crisps, ready meals and sandwiches.
What to look for on the label
Look for "suitable for vegetarians", "microbial rennet", "vegetarian rennet" or a halal logo. Paneer and many cream cheeses are set with acid alone, without rennet. Cottage cheese often has a little rennet, and ricotta is often made from the whey of rennet cheese, so check those too. Parmigiano Reggiano and Grana Padano always use calf rennet.
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.