Bangkok is a mecca for shopping enthusiasts. There is a wide range of places to go, from open-air markets with copies to hi-class shopping malls with designer clothes.
Although Bangkok hosts many different malls all over the city, the majority of them is to be found in and around the Siam Square area. This is where most tourist find themselves as well. There are extravagant malls like the Emporium and there are malls that can keep you busy for many days, like the Mah Boon Kroong Shoping Mall (MBK).
There are plenty of markets for you to choose from. One of the more famous ones are the Patpong Night Market, that grows up from nothing every night as Silom's business district turns to a red light district. There is an other less known night market called Suanlum. It is a much more 'clean' market where you also will find locals shopping (cheaper than Patpong).
An other famous one is the Chatuchak Weekend Market and as the name implies, it is only on the weekends. It is a very large market with everything you can possibly think of. A tip; bring a water bottle, as it gets very hot there and it can be hard to find something to drink at times.