WebRequest is an anstract class which has a factory method Create that
depending on the URL passed in creates an instance of a concrete
subclass. Whether you need or want
  HttpWebRequest httpreq = (HttpWebRequest)WebRequest.Create(strUrl);
instead of
  WebRequest req = WebRequest.Create(strUrl);
depends on your needs and on what kind of URLs you pass in. If you only
pass in HTTP: URLs then the former allows you to access the properties
and methods the subclass HttpWebRequest implements in addition to those
defined on the base class WebRequest. But if you passed in a FTP: URL
then the attempt to cast to HttpWebRequest would fail.
The latter is generic and won't fail on any of the types of supported
URLs but of course without casting to any subclass you can only access
the properties and methods the base class defines.

To add to Martin's comment, you also have access to more HTTP specific
properties such as Content Type, Cookies, etc.