There are various Meta Tags that are evaluated by search engines but there are really only 3 main 'Meta Tags' that you should focus on.
The 'Title' Tag : This should be 40 – 90 characters long and should be 'a heading style' sentence containing the top keyword(s)/phrase(s) for that page. Do not repeat the same keywords more than 3 times. - ideally you should focus on 1 or two keywords/phrases per page.
The 'Description' Tag: This should be 150 – 200 characters long or not over 20 words. Again this should contain the top key phrases for the page but not more that 3 to 4 times.
The 'Keywords' Tag: Max. 1,000 characters or 50 words, however it is better to stick to relevant keywords/phrases for the page rather than just stuffing this tag with as many words as possible.