Intensely flavored curry pastes are a staple of Thai cuisine. These moist, concentrated blends of chiles, spices, and aromatics like lemongrass, lime leaves, shrimp paste, shallots, garlic, ginger, and cilantro are stirred into coconut milk or broth to make the sauce for all sorts of curry dishes.
There are several styles of curry pastes. Green curry paste gets its color mainly from fresh hot green chiles like serranos; they tend to be the hottest curry pastes. Red curry pastes are made with dried red chiles; they're pretty fire too, but not quite so much as green pastes. Yellow curry pastes are colored by turmeric and Indian-style curry powder; their spice level is relatively mild. Panang curry paste is similar to red but with the addition of ground peanuts. A fifth style, called Massaman (or Masmun) curry paste, is more Malaysian in influence; it's also relatively mild.
It's better to substitute one type of curry paste for another than substitute Indian-style curry powder for the paste altogether; the flavors are dramatically different. You can also make your own curry paste, though the ingredients are often harder to find than the paste itself.
Look for jarred curry paste in the Asian foods section of your grocery store (Thai Kitchen is a reliable and widely available brand). An Asian market will give you a wider selection, including fresh and frozen curry pastes that have more flavor and punch.
Keep curry paste tightly wrapped and sealed for up to a month in the refrigerator, or up to three months in the freezer.