meta tagging..

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hello friends.. I want to have a clear idea of meta tagging...please share your knowledge.. what should I write in the description tag...and how long should I write...? what is a title tag...is it only content the site name...or there should be some more things like what is the site for..? what about the keyword tag? how much keyword I can put there?is there any Limit in that? please share your knowledge with me..It will be batter if you can give me a little example too...thanks in advance..