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

Question

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)

Answer 1

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}}">