[Tugindia] what are .cls files?

[Jagadeesh Bhaskar]

i have a lot of .cls files that i know are related to Latex........what is 
its use?
when i tried to include ".cls" file names in \usepackage{} as using ".sty" 
files,it gave error saying that ".sty" file is missing!!

is it that i can do nothing with these files??
please help me out!!

thanking in advance for replies,
jagadeesh..

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