Are you a GCSE student, or an A-level student...?
University applications are for students who are in the process of completing their A-levels. For these applications, refer to PQ's post, which explains the differences in those qualifications you noted.
If you are looking to apply to 6th Form Colleges as a GCSE student, you should look at the "16-18 Choices" section of the UCAS website and search in that; you won't normally be applying to university until Year 13.