What are the important meta tags I must put in my website?

There seems to be huge number of meta tags you can set. I'm guessing that some of them are more important than others.

What are the most important metatags to supply and why?

Do I even need meta tags? (looking at the stackoverflow homepage there are none)

I realize this question is old, but it's still a top google hit, so I thought I would give an updated answer that includes popular social media sites.

I generally have four groups of meta info:

Regular Meta Info - used by search engines and browsers

<title>{{pageTitle}}</title>
<meta charset="utf-8"><!-- html5 version of http-equiv="Content-Type"... -->
<meta name="description" content="{{description}}">
<meta name="keywords" content="{{keywords}}">
<link rel="author" href="https://plus.google.com/{{googlePlusId}}" />
<link rel="canonical" href="{{pageUrl}}" />

Facebook Meta Info - used by Facebook when someone shares your url

<meta property="og:url" content="{{pageUrl}}">
<meta property="og:image" content="{{imageUrl}}">
<meta property="og:description" content="{{description}}">
<meta property="og:title" content="{{pageTitle}}">
<meta property="og:site_name" content="{{siteTitle}}">
<meta property="og:see_also" content="{{homepageUrl}}">

Google+ Meta Info - used by Google+ when someone shares your url

<meta itemprop="name" content="{{pageTitle}}">
<meta itemprop="description" content="{{description}}">
<meta itemprop="image" content="{{imageUrl}}">

Note: you don't really need these, Google+ will fall back to the Open Graph tags that Facebook uses.

Twitter Meta Info - used by Twitter when someone shares your url

<meta name="twitter:card" content="summary">
<meta name="twitter:url" content="{{pageUrl}}">
<meta name="twitter:title" content="{{pageTitle}}">
<meta name="twitter:description" content="{{description}}">
<meta name="twitter:image" content="{{imageUrl}}">

I'll use my same answer from this question:

A few years back, meta tags were important to search engine optimization. However, they've been abused and are generally ignored by almost all search engines (including Google, Yahoo and Live search. Excuse me: Bing).

The most important tags for SEO that you can include in your (X)HTML are the <title> and <meta name="description"...> tags.

• <title> should generally be what you'd want the search engine to name your page in it's listing.
• <meta name="description"...> can sometimes give the search engine a basic idea of how to describe your page when indexing it.

However, using these two tags will not necessarily make a difference in increasing your site's visibility on a search engines listings. For more information on that aspect, Google has a nice section on SEO on their site.

Update May 2015: Still being a very popular answer, I recommend you look at @alden's answer as it is much more updated then my own (6 years old now)

Original answer follows:

---

<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8"> 

To tell the browser what the content type and encoding is

<meta NAME="ROBOTS" CONTENT="NOODP">

See http://www.mattcutts.com/blog/google-supports-meta-noodp-tag/ for why

<meta name="description"...>

Obvious

<meta name="keywords"...>

Google do not use this, but other search engines may

<meta HTTP-EQUIV="Expires" CONTENT="Tue, 20 Jun 1995 04:13:09 GMT">

If you know when you like this page to be expired from cache

The most important meta tag you should use is:

<meta http-equiv="Content-Type" content="text/html; charset=utf-8">

adjusted to suit. This ensures that, if you ever need to transmit that HTML document via something other than a web server (e.g. working on it locally, sending as an attachment), the user-agent is aware of its mime-type and character set. Just make sure your server headers agree.

The best SEO is a website that meets the needs of its target users effectively. To do that you need to have a site that has something in it that people can use, want, or need. This is value and its the most important thing. It can be anything from good articles to entertaining video or a useful download. Tags support good content but cant replace it for SEO. If the content is good enough no tags are needed. Trust me when I say, if your site has something, anything people want or need, that they can't get just anywhere, your site will do well in all regards, tags or no tags. If your site is boring and has no value it will fail, tags or no tags.

the description metatag will be used by google as description of your site. I think it's the most important.

I think that the verify-v1 meta is important. It makes possible to use Google Webmaster tool

GoogleBot ignores meta information only trusting the page content itself rather than content descriptors. So, the usefulness of certain tags may be limited.

I would make sure you have a title, description, keyword, and revisit-after tag in your web pages. Although Google doesn't weigh keyword tag as much, it may still be used by smaller search engines.