How the metadata is interpreted by search engines
and how to effectively write metadata.
The <title> meta tag
The content of the meta tag "title" is what will appear at the top of your browser's window or tabs. It is important that this is an accurate reflection of your site's content but also that it is short enough to read at the top of pages. The title and description tags are favored by search engines as they provide very good insights about the content of your website. The title tag will also appear for readers in search results.
You could also think of the title tag as being similar to the tabs. Both guide the reader by giving him a short description of where he is on the site. Each title should be different and unique for each page of your site.
The <Description> meta tag.
The description (of your page) is what your potential readers will see under the title when Google lists your site in its search results. The description should be two or three sentences that are a simple but detailed description of the contents of your page. The purpose of the description is to let the reader confirm that what they have searched for is actually what will be presented to them when they visit your page. Google also uses your description to make this same confirmation so you should be attempting to use the most important keywords that appear in the content of your page.
Think about how readers will read a search page in Google. They will read very quickly, scanning the titles and one or two keywords in the description. It is wise to make sure that the most important keywords come near the beginning of the description tag so that they are not missed by readers.
Here again you should be careful to avoid superlatives. (the best, wonderful, most, excellent, etc) are all terms that Google and readers are suspicious of.
The <Keywords> meta tag
Often overrated, most probably not even such an important marker for Google, the meta keywords should still be documented. For each page, choose the 5 most significant words in your page, and place them here in order of importance.
Avoid using <keywords> that are not used in the actual content of your page.
The "name of this file on server" tag.
is not actually a meta tag but it can be grouped here because it also helps readers and Google reference and understand your site.
In the address of this very page:
"how-to-define-meta-tags" is the name of the file
This should be much like the title, it needs to be a description of the content of the page but it also needs to be very short. The name acts as both a way point for your readers in the address of you page (url) and a reference for Google to understand the content of your page.
Note: Once you have a file name you should be careful of changing it. If you change the file name of any of the pages of your site, you also change its address. Links coming into this page will be lost. It is best to choose a file name carefully and then leave it.
Note 2: for the first page of a site, the name of the file is always [index.html].