Feta is a rindless, white cheese aged in brine originally from Greece. It's great crumbled onto all types of salads, stuffed in vegetables and pies, folded into casseroles, blended into dips, baked with oregano and olive oil, and most commonly, just sliced into slabs to be nibbled on throughout the meal.
Feta can range widely in texture and flavor; some are soft and moist, others hard and dry. Some are crumbly, others more creamy. Some are salty, others more tangy. The differences come from how the cheese was made, whether it contains sheep's, goat's, or cow's milk, and how long it was cured. Traditional Greek feta usually consists entirely of sheep's milk, although it may contain up to 30% goat's milk. But feta is made in dozens of other countries too, including France, Spain, Israel, Australia, and the United States, where it's mostly made with cow's milk.
Substitute farmer's cheese or Mexican queso fresco.
Imported and artisan-crafted feta made of sheep's and goat's milk can be delicious, but you may need to visit a specialty store to find them. Fortunately, supermarkets carry plenty of feta, some of it quite good. Whether it's crumbly, hard, and dry, or soft, moist, and creamy, good feta should taste and smell fresh. Compared to most cheeses, feta has a pronounced but pleasing acidic tang. If it smells or tastes overly sour, or if it has developed a peppery aftertaste, it's probably over the hill.
Buying feta in whole blocks, bricks, or wedges makes sense for the same reasons that you buy Parmesan in big chunks: it stays fresher for longer, it doesn't dry out, and its flavor packs more punch. Also, it gives you more options. Sometimes you want to slice a thick slab of feta and other times you need large crumbles. Finally, feta sold in whole pieces is often—though not always—a sign of a better-quality feta.
You can make a crumbly feta more creamy by cutting the brine with milk—about 1 or 2 tablespoons per pint of brine will do. It takes a few days for this little trick to take effect. You might need to try a few batches before you get the amounts of salt and milk just right, but the cheese won't suffer in the meantime.
Store feta in the refrigerator in the brine it came in. If there is none, make your own brine (add a few hefty pinches of salt to a pint or more of water) and store the feta in a plastic container. A large chunk of fresh and properly stored feta should last up to three weeks.