Meta Tags

Meta tags contain information that is viewed by the search engines and should appear below the Title within the HEAD section of the page source code.

Not too long ago meta-tags were seen by many as synonymous with SEO, this despite the fact that their importance to ranking had long since waned to the point that some SEOs had actually stopped using them altogether.
That proved a bit premature as nowadays the lack of a page specific description tag can effect how the page gets indexed by Google.

These days the main ones to consider are the "description" and "robots" tags with to a lesser extent the "keywords" one, there's also some recent ones to stop search engines using Directory titles.

Description Tag

The description tag often shows up with a search engine listing and should be used to entice the reader to click the link as well as "complementing" the Title.
For example the Title of this page is "optimising meta tags" while the description is "Optimising the page meta tags - which ones and why.", a bit short and repetitive to be ideal perhaps, but you get the idea.

These days the absence of or use of a generic description tag can have the page placed in Googles' supplementary index, this is particularly likely if the page also has little original content and low PR.

Robots Tag

The robots tag is used to tell a spider what to spider on the page, they are requests rather than commands so can be ignored by "rogue" spiders.

The two most common directives are index which tells the robot whether to index or not, and follow which tells it whether to follow the links or not and can be used in the following combinations:

<META NAME="ROBOTS" CONTENT="INDEX,FOLLOW">
<META NAME="ROBOTS" CONTENT="NOINDEX,FOLLOW">
<META NAME="ROBOTS" CONTENT="INDEX,NOFOLLOW">
<META NAME="ROBOTS" CONTENT="NOINDEX,NOFOLLOW">

So, in the last example the robot is requested to neither index the page content nor follow the links on that page.

Keywords Tag

For further reading on meta tags we recommend: