Questions about meta tags and title elements routinely come through SEO Chat. Here are the largely accepted answers for these elements.
Title Element (some call it the Title Tag): Exceptionally important. Use it for page description, brand recognition, keyword placement. Want to know how many characters? Go count yourself in Google results. Google can read more, but places more prominence on the keywords at the start. Do'nt stuff unnatural keywords in a title element, make it concise and concrete.
meta description tags: IMO no benefit to ranking. IF it's off topic and unrelated Google may opt to use your content to generate their own description (meta description tags' only use are to provide a snippet of the content in the search results). We can almost certainly conclude you don't get a ranking benefit from having the tag.
The only benefit is that it may provide the impetus for more clicks then related result with an unclear description.
However most disagree whether this is actually true since tests are inconclusive. From my perspective it DOES help a small minority and is therefore beneficial for said reasons above. If you, however, have targeted content that will do well as a description, no need for the tag.
From my tests on eye-tracking and CTR the TITLE element is usually what people will view and not the description tag. Having said THAT we still have evidence that descriptions induce clicks for the top three results.
meta keyword tag: Useless. Ignore.
There are some reputable SEOs who continue to write that keyword density is relevant. Fact is, there is no magic % you need to follow. You don't start writing content with a keyword density in mind, but rather, your content winds up with a final keyword density score (which likely plays little to zero role in helping your rank.)
If you have done the correct market research then you know what your target market wants to read. From there you will be writing relevant material based on demand and not based on what you think the best 'keywords' are.
Moral of the story? Write natural and value added content to your target market and never worry about keyword density. If you ever start with 'keyword density' or hear someone talk about it then run in th eother direction, these individuals at best don't understand or practice proper business analytics/research.
At the end of the day it's about your content. if you pay attention for LASTING and VALUABLE content to your target market then you will NATURALLY incur keywords throughout the document/page.
Write for people, optimize for engines, write great content for people, lasting content, valuable content. Stop WASTING time on densities. It's so 2003 .
The only other meta tags that can be useful (there are a lot of meta tags that people stuff in, few are required, even fewer have any benefit) are:
-Content Declaration (usually UTF-8)
-Nofollow NoIndex (for an added layer of protection to prevent bots from visiting/indexing pages)
-Canonical (again, added layer of protection to clarify what pages Google should treat as 'correct')
-NOODP (if your descriptions are pulled from DMOZ)
Other than the first the rest are really unnecessary and none have an impact on your rankings.