Thailand has a plethora of accommodation in every price bracket. Always take a look at the room or better still several rooms, sometimes owners offer not the best/cheaper rooms first before agreeing a price. In smaller establishments also do ask for the agreed price in writing to avoid problems during check out.
The best prices 30%-50% off rack rates for accommodation can be found during Thailand's low season, which is during May - August, which not surprisingly also coincides with the region's monsoon season. The peak season is during December - February.
The prices listed are average for the country, and vary depending on the region and season. Smaller provincial towns will not have fancy hotels or resorts, while on popular island beaches it may be hard to find something cheaper than 300-400 baht even during the low season.
Guesthouses are usually the cheapest option, basic ones cost 100-200 baht per room per night 100 or less for a dorm bed. This gets you a room with a fan, a squat toilet often shared, shower shared or private and not much else. Better guesthouses, especially in towns with significant amount of foreign guests, have more amenities European style toilet, 24h hot shower, bigger room or even balcony, free wi-fi internet, sometimes TV, everyday room service, fridge - with price, subsequently, in the range 200-500 baht. This makes them close to Thai hotels - however, the difference is they're more oriented to the Western clientele, and as such often offer various tours sometimes overpriced, computers and/or in-room Internet access, or even have a ground floor restaurant.
If you're satisfied with the guesthouse of your choice and plan to stay there for more than several days especially during the low season or in the places with abundant accommodation options such as Chiang Mai - ask for a discount; this may be offered not everywhere, but if it is - even weekly rate may be 25% less or so, and for monthly rates it's not uncommon to be twice or more cheaper. Normally, you'll have to pay for the entire period asked, but note that if something changes and you have to check out early - money refunds are not customary in Thailand. As such, if an early departure is possible but unlikely enough to pay a week/month in advance - you should discuss this option with the owner/manager beforehand.
Hostels are something not typical for Thailand. The reason is obvious - given the abundance of the budget accommodation in Thailand, and that hostels are unknown to Thais and, as such, purely Westerner-oriented, the price for a private room in a guesthouse will be almost the same, or even cheaper, than for a dorm bed in a hostel! You may get a bit more Westernized and hotel-like interiors, but is it worth the lack of privacy? Anyway, if you do insist on staying in a hostel, you can find some in the big cities by looking on the corresponding global websites. Don't expect to find them just by walking by the streets, though.
Thai hotels start around 200 baht and go up to around 800 baht. The upper end of this range will be air-conditioned, the lower end will not. The primary difference is that with a hotel room, your bathroom should be private, bed linen and towels should be provided, and there may be a hot shower. The guests are mostly Thais. TVs are available except in the lower end; Internet access, though, is less likely to be present than in guesthouses; and is even less likely to be complimentary and/or in-room.
Tourist hotels are generally around 1000 baht and offer the basics for a beach vacation: swimming pool, room service and colour TV.
Boutique hotels, 2000 baht and up, have mushroomed during the past few years, they provide a limited number of rooms 10 or less and more personalized service. While these can be excellent, quality varies widely, so research is essential.
Business and luxury hotels , 4000 baht and up, offer every modern amenity you can think of and are largely indistinguishable from hotels anywhere else in the world. Some, notably Bangkok's The Oriental, The Sukhothai and The Peninsula are among the world's best hotels. The most luxurious resorts also fall in this price category, with some of the very best and most private adding a few zeros to the price.