Document Files Extensions - What are document files and how do I open a document file?

What are document files and how do I open a document file?

File Type Category File Format
.aww Document Files Ability Write Document
.btd Document Files Business-in-a-Box Document
.ec4 Document Files EPSON Print CD File
.mcdx Document Files Mathcad Prime Document
.mws Document Files Maple Classic Worksheet
.odp Document Files OpenDocument Presentation
.odt Document Files OpenDocument Text Document
.sdw Document Files StarOffice Writer Text Document
.sgm Document Files SGML File
.t2k Document Files Teach2000 Document