Hangul (aka Hangul Word Processor or HWP) is a word processing application from Haansoft, and is very popular in South Korea, mainly due to the way it has been designed for the Korean written language.
Text documents created in Hangul are saved with the extension HWP. Any HWP files created with Hangul '97 or before can be opened with OpenOffice. However, .HWP files created after this version are not compatible due to the changes in the file format's structure.
Most recent versions of Hangul are able to save documents in Microsoft's .DOC format.