Meta tags - Title Description and Keywords

Definition : HTML entities placed within the HEAD section of a webpage. It is invisible to the visitors. The most relevant and famous META tags are KEYWORDS & DESCRIPTION tags. These tags are not very useful as these tags are used by some search engines only.
Examples of tags : KEYWORDS & DESCRIPTION
META name="KEYWORDS" content="Keyword Phrases 1,2,3...."
META name="DESCRIPTION" content="Description"...try to keep the length of description upta 200 character.
Some of the other META tags are :
a) META name="Author" CONTENT="Author Name" - You can use it to indentify you as an Author of the website.
b) META http-equiv="Content-Language" CONTENT="Language" - This tag is used to identify about the lanuage of the website. If you uses contents in english for your website the you can use abbreviation "EN" in content.
c) META name="copyright" CONTENT="statement" - This tag lets know to the visitors that your contents is copyrighted.
d) META name="expires" CONTENT="Date" - If your site will be expires on any particular day then you can use this tag to inform the visitors about it.

Important Points :

1) Title tag should not more than 5-15 words, maximum 2 commas.
2) Description should be upto 200 characters means 25 words. Some search engines read more than 170 characters and some reads about 130 characters like google.
3) Keywords should be 1000 characters in length, approximately 150 words.