Ok, so.. I only will be having a high school diploma to work with here. When I've been looking at qualifications and such for entry into a college, I find quite a bit of an 'undergraduate' as a qualification or 'At least five GCSE passes at Grade A*-C.' or 'A minimum of 240 tariff points with at least 2 A2 (A-level) qualifications. At least one A2 qualification must be an art and design related subject supported by passes in four other subjects at GCSE level (grade C or above.' Ok, what the hell is that? Seriously, might as well be speaking another language as I have no idea what that is. Another thing that is not helping is I've been very sheltered from things since I've been schooled at home since the 6th grade (aka, the start of middle school).
Okay, a quick (basic) breakdown of the qualifications:
GCSEs are 2-year courses taken in several different subjects between ages 14 and 16 (English, English Literature, Maths and Science are compulsory, plus most students also take a language, a humanity subject, a technology subject and one or two others). A C grade is a pass at GCSE, so that's why the universities ask for grade C and above.
In terms of US education, if you have a US high school diploma, that is generally considered to be equivalent to '5 GCSEs at grade C or above'.
A levels are 2-year advanced level courses taken in 3 to 5 subjects between ages 16 and 18 and they are the requirements for getting into a UK university. Students take the subject(s) that they are considering studying at university as they may well need to have in depth knowledge of that subject to get offered a place (i.e. the requirement of having an A level in Art in order to get into an art-related university course). The first year is called AS level and the second year is called A2 level... most universities require that you have taken both years of A level to get offered a place
The A level points system is a little bit confusing, but basically each grade is worth a certain number of points: A* = 140, A = 120, B = 100, C = 80, D = 60, E = 40. So, if you need 240 points, this could be made up of 2 A grades; or 2 B grades and 1 E grade; or 3 C grades etc.
In terms of US education, A levels, are approximately equivalent to either AP classes in high school or possibly the first year or so of college. The UCAS website has a table of equivalent A level points for US AP grades on this page
http://www.ucas.ac.uk/students/ucas_tariff/tarifftables/ (4th table down).