What does the symbol ? mean in a URL?

Question

What does the symbol ? mean in a URL?

Answer 1

That portion of the URL (ie. after the ?) is known as the query string.
http://en.wikipedia.org/wiki/Query_string

It is used to pass parameters into web applications.

For example, in ASP.NET I might have an .aspx page like so:
http://example.com/myapp/default.aspx

Inside my codebehind for that page I can look for the presence of any query string parameters:

string paramValue = Request.QueryString["param"];

So if someone visits my page with the URL of http://example.com/myapp/default.aspx?param=abcd
Then the value of paramValue will be "abcd".

Answer 2

RFC for http protocol, section 3.2.2 http URL

"?" - is delimiter between "absolute path" and "query"