Use this for GET values:
Request.QueryString["key"]
And this for POST values
Request.Form["key"]
Also, this will work if you don't care whether it comes from GET or POST, or the HttpContext.Items collection:
Request["key"]
Another thing to note (if you need it) is you can check the type of request by using:
Request.RequestType
Which will be the verb used to access the page (usually GET or POST). Request.IsPostBack
will usually work to check this, but only if the POST request includes the hidden fields added to the page by the ASP.NET framework.