As many of us already know, Thai curries are made from a base of paste, not powder. You can make your own curry paste with a mortar and pestle or blender (food processors don't grind as finely, so aren't recommended), but it is difficult to achieve the fine consistency of a commercial curry paste. My Thai friends here in Toronto just laughed at me when they saw me trying and told me to go out and buy my curry paste in Chinatown - "It's just like in Thailand!" - and so that's what I do now.

I have seen curry paste for sale in cans, jars, plastic tubs, and plastic envelopes. No difference. The thing that does make a difference is the brand, and different people have different tastes. I prefer Mae Sri brand, which comes in cans, but you should try them and see which one you like best.

The basic ingredients for curry paste are:

But there are several different types of curry paste, by and large distinguished (in English translation, at least), by colour. These are:

  • red curry paste (gaeng phed, "spicy curry"): The reddish-brown colour comes from the dry red chilis. Usually the hottest of the pastes. It is also one of the most versatile; besides being mixed with coconut milk to make a soupy curry, it's also used in stir fries with yard long green beans, and as a flavouring for Thai fish cakes.
  • green curry paste (gaeng kheow wan, "sweet green curry"): The name is misleading, as this too is a hot one. The green colour comes from fresh hot green chilis, and sometimes fresh herbs such as coriander (cilantro) and basil. This paste is traditionally combined with round green eggplants, pea eggplants, and chicken or shrimp. My favourite.
  • yellow curry paste (gaeng gali or gaeng khaek - khaek means "guest", but is usually used in reference to South Asians, so this is "Indian curry"): The mildest curry. The yellow colour comes from the addition of curry powder, particularly turmeric. This curry can be found in Malaysia and Indonesia as well, and in Thailand is usually prepared with coconut milk, chicken and potatoes.
  • massaman curry paste (gaeng massaman): This is kind of a cross between red and yellow; it is based on the red, but with added dry spices like cinnamon and anise. It usually has coconut milk and chopped peanuts in it.
  • orange curry paste (gaeng som, som is "orange"): This one is a little different; the curry itself is usually filled with seafood and cabbage, and the taste is rather sour. No sweet coconut milk in this one. Less common in the west, as it's more to Thai than farang tastes. Give it a try, though, if you like.

Take it easy with curry paste until you know the spice level of what you're working with; you can always add more if it's not hot enough, but it's difficult to cut the heat if you over-estimated the amount you can handle, though adding a little sugar or lime juice might help a bit. I use a tablespoon or two for a dish, no more, and it has a pleasant burn. Store extra paste in the fridge for a month or two, or in the freezer if you think you can't use it all up.