To make this simple: GSM operates on four different bands, two in the Americas, two in the rest of the world (850/1900 and 900/1800) respectively. If you only have one of the bands of the region, you may well experience reduced coverage (see http://www.gsmworld.com/roaming/gsminfo/cou_in.shtml for a list of coverage in India). All GSM phones operate on two, three or four bands. A quad band phone works everywhere and is your safest bet. A tri-band phone is customized for North America or the rest of the world. One for NA will not have 900, one for the rest of the world will not have 850. A dual band phone will have only 850/1900 or 900/1800 depending on where you get it.
So, ask youself first: do you need have to some coverage in North America? If so, rule out European/Asian dual-bands (they tend to have few features anyway). Do you want some NA coverage? Then you can get a European tri-band that has 1900 but not 850 (do not get a US tri-band like the SonyEricsson T637 as it has the 850 but not the 900--I would not recommend getting a phone without 900 if you going to travel around a lot in India). If you want as good NA coverage as possible too, get a quad band.
Your phone does need to be unlocked, but you can usually do that easily overseas for less than $20. If you order a phone from ebay, wirelessimports or similar place it will come unlocked. You will only get a locked phone if you buy one from a provider (T-Mobile or Cingular--in principle they will unlock the phone for you but only after you've had it for a while, and this does not mean you are out of the contract), and then you are stuck with a contract though you can knock a lot of the price off: for example, the SonyEricsson W600i (which is a quad band with a 1.3 megapixel camera) is between $350-400 unlocked, and less than $150 with a Cingular contract.
Hope this helps.
