Every search engine has it’s own rules to process the html tags inside the head tag. Following next are some some general rules.
The <Title> Tag
First, the title of the html page should be relatively short and describe the page content accurately. Wherever possible, try to include keywords but without distorting the true purpose of the title.
It is one of the most important elements of SEO when it is used properly. A website can increase organic search traffic for each page by using an appropriate keyword within this tag, but this is only effective though when the actual content of a page is about that specific keyword. Search engines are indexing the content of the page as the meta tags and they can easily figure out whether or not the title tag is appropriate for the content contained on a specific page. In order to get full benefit of this tag, the keyword used within it should appear in its exact form within the content of the page. This is the best way to relay to the search engines exactly what a page is about and what keywords it should rank for within the search engine result pages.
The domain name shouldn’t be repeated in the title, it is often considered as spam. Also, it should not be used the same filename as the title tag or the same filename as the domain name, it is often considered as spam.
The title should not be any longer than 70-100 characters including spaces. (Google – DMOZ)
The title should bot be any longer than 60 characters. (scrubtheweb.com)
The title should not begin with the domain name, it is often considered as spam.
The <Meta Keywords> Tag
The meta keywords tag is the least important metatags because the majority of current search engines no longer support these tags and they place little to no importance on them, but it is good to have them filled in on each page. Use the description and keywords metatags in the head of each web page and make these tags different on each web page. The search engines does not like duplicate metatags.
First of all, keep the keywords as descriptive as posibile. It should not used words that are not present in the body of the page (for example: do not enter movies in the keywords tag when the page is only about books). Redundant characters will not hurt overall results, however words after the first 300 characters rarely help in any way.
The first keyword should be the main keyword that the content is based around, this keyword generally appears within the title of the article or page content. The second keyword should be the second most important keyword that the content is based around. Then you should list the next four important keywords or variants of the main two keywords. This has the potential to help a search bot to understand what the content of each page is about.
The tag should not be any longer than 378 characters (searchenginewatch.com referring to Google).
The tag should not be any longer than 268 characters (AltaVista).
Start all keywords with capital letters. (Relevant only on alphabetical listings)
Separate keywords with the “, ” (comma, space) character combination. (Most search engines use either character as the separator ).
You may use phrases as well, but it should not be used any word, not even within phrases, more than 3 times. It is often considered as spam.
The <Meta Description> Tag
This is a very important element of SEO, it allow a unique opportunity for websites to convince searchers to visit their sites and a particular page on the site. First of all, keep the description as descriptive as posibile, but the descriptions on some web pages are often a snippets of content that appear below the clickable titles of a search listing. Creating high quality meta descriptions can improve click through rates in the search results, also creating a poor description can be one of the quickest ways to get passed over by searchers within the search results.
It should not be any longer than 100 characters (Google).
It should not be any longer than 25-30 words (DMOZ).
It should not be any longer than 150 characters (scrubtheweb.com).
It should not be any longer than 200 characters (searchenginewatch.com referring to Google).
The source is here.