The organization which processes candidates' applications for a place at a university or other higher education institution to study for a first degree, a university diploma, or a Higher National Diploma. It was formed in 1992 as a consequence of the merger between the Universities Central Council on Admissions (UCCA) and the Polytechnics Central Admissions Systems (PCAS), which followed as a result of the Further and Higher Education Act 1992, which granted polytechnics university status. UCAS is also responsible for the work of the Graduate Teacher Training Registry. Applicants to higher education who have yet to sit their Advanced Levels (A levels) will normally be offered a place which is conditional on their achieving the required number of A level points. Those whose results fail to gain them their conditional place, or those who have waited until after their results to make their application, may go through the UCAS clearing process, which aims to match suitable applicants to places which remain unfilled.
http://www.ucas.com/students/afteryouapply/clearing An explanation of how the clearing system works.