What is a method group in C#?

I have often encountered an error such as "cannot convert from 'method group' to 'string'" in cases like:

var list = new List<string>();
// ... snip
list.Add(someObject.ToString);

of course there was a typo in the last line because I forgot the invocation parentheses after ToString. The correct form would be:

var list = new List<string>();
// ... snip
list.Add(someObject.ToString()); // <- notice the parentheses

However I came to wonder what is a method group. Google isn't much of a help nor MSDN.