Ko Chang is an island (Ko means "island") in the eastern part of Thailand in Trat province near Cambodia. It is Thailand's second largest island, and the entire island chain comprises Ko Chang Marine National Park, so, you have to pay 200 baht to get in (unless you are Thai).
Naturally, since it is an island, you need to get there somehow. Bangkok air has an airport in Trat, near the island, and they also operate a shuttle bus to the island which will also pick you up when you make the return trip. Alternatively, you can take the bus from Bangkok, which would take 6-8 hours to get to Trat, after which you would make you way to the ferry. It's pretty easy.
Most of the accomadations consist of "beach huts". My beach hut had air conditioning, a t.v., refridgerator, etc. This isn't cheap - I think it was about 2000 baht ($65) per night. More primitive, "traditional" beach huts are also available, and there is huge range of options to choose from, so your milage may vary. It might be worthwhile to book ahead of time (I never do).