A surcharge is one type of overprint. A surcharge stamp is a postage stamp overprinted to alter its face value. There are a few reasons for a postal authority to produce surcharge stamps. For whatever reasons, surcharge is always done on surplus stamp stocks.
Pos Malaysia (previously known as Jabatan Perkhidmitan Pos) has not done any surcharge stamp as far as I remember. I like surcharge stamps as these stamps carry extra postal information. Thai Post has been doing surcharge stamps from time to time. Below are just a few surcharge stamps from Thai Post.
the original stamps were issued on December 14, 1994 with a face value of 2+1B. The stamps have been surcharged to 5B
the original stamp was issued on December 5, 1997 with a face value of 6B. The stamp has been surcharged to 10B. 
the original stamp was August 28, 1997 with a face value of 6B. The stamp has been surcharged to 15B. 
the original stamp was issued on July 19, 1998 with a face value of 6B. The stamp has been surcharged to 15B
