Text Files Extensions - What are text files and how do I open a text file?

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

File Type Category File Format
.lst Text Files Data List
.man Text Files Unix Manual
.nfo Text Files Warez Information File
.p7s Text Files Digitally Signed Email Message
.rtf Text Files Rich Text Format File
.txt Text Files Plain Text File
.wpd Text Files WordPerfect Document