title: Explain the ' notation in F#
body: <p>I'm learning F#, when I type any code in Visual Studio and run them on <em>F# Interactive</em> it shows me things like</p>
  
  <pre><code>val foo : x:'a -&gt; 'a
  </code></pre>
  
  <p>I imagine this mean that <code>foo</code> is a function that receive a paramater <code>x</code> of a type, and returns a value of same x's type.</p>
  
  <p>But <strong>What does that <code>'</code> mean?</strong> Many functions show such thing on intellisense too.</p>