The DTD statement. This should be the first tag of the head section of your code. The Document Type Definition Statement allows faster and deeper indexing with Google, shortening the time your site will be in the trustbox as well. HTML 4.0 or 4.01 should be the standard, and for most cases, the Transitional type should be used. The title, the most important. Why? Because there are 3 elements in SEO: the listing (were title is the main element) the click-through (were title is the main reason) and conversion (which is the object of all optimization work) Also, if that is not reason enough (it should) it is the single element that gests indexed and used to list the link text in the Google SERPs (Search Engine Results Pages). An average of 7 to 8 words length is optimal. The description. Used by Google to create a text summary in describing the page if available, so make sure the content of this tag is friendly to the searcher, not the search engine. If you are in a competitive market, this tag is not taken into account, but you should have it for your visitor. An average length of 150 characters is good. The keywords. Google actually uses this tag against you, by that meaning it is used as a spam check point for the page content. Put your main keywords across the content. Here use an average of 200 characters. Make sure you are using different sets of keywords per page, in other words, that they are unique to each particular page.
I hope you would refer newb's to better, more comprehensive guides than this post. Prakash, Google doesn't use keywords in rankings and title tags, YES, they are important and influential in rankings BUT I would say back links are the "most important" as you can have perfect title tags, great on-page copy, and still note rank with 0 back links.
Meta tags is just one of the seo activity there are lot of other seo activities that are need to be performed on the site to get good results. I agree with Brad.