Air Asia are a low fare airline originating in Malaysia.
Air Asia offer a selection of domestic and international flights within Thailand, Indonesia, Macau (Greater China), Singapore, Philippines, Cambodia and Vietnam.
Air Asia was heavily in debt when a former Time Warner executive left his job and took up the challenge of turning the company around. Tony Fernandes has now turned Air Asia into an award winning airline by using Ryanair as a template for a low cost airline.
Air Asia operate a fleet of Boeing 737-300s.
As with most low cost airlines, Air Asia do not provide free meals or drinks, but there is a reasonably priced catering service (reasonable for airlines).
You can book online and I find this to be the most convenient method. You can also book Air Asia via walk-in airport sales stations, sales offices and travel agents.
The way the fare system works, is that seats start out cheap and get more expensive as they are filled. Obviously this means that you should try to book as early as possible, because you are more likely to get a cheaper flight. This also means that less popular days or times will likely be cheaper options. Also note that any holidays are likely to be pretty busy and getting a cheap flight, or even a flight at all may be a problem.
Note that on some of their flights there will be a mad rush for the check-in desk when it opens.
Air Asia are definitely worth checking out if you are travelling around Asia.