Basically, UK universities just don't work in the same way that US ones do.
What UK degree course will you be taking?
The reason I am asking this is so I could perhaps take three years instead of four years to complete my degree.
If you're doing a UK degree, it should only take you 3 years anyway, unless you are doing a specific 4-year degree (undergraduate masters, languages degree with an extra year abroad, normal degree with year-long placement in industry or a study abroad year). Undergraduate degrees are generally 3 years in length with basically no flexibility on how you long take to complete it - you start in September of one year and finish in June/July 3 years later (unless you fail one of the years and have to retake the whole year; you have to take a year out due to serious family/personal problems; or the other reasons I mentioned above).
Almost all UK university undergraduate degrees are pretty structured - you only study one subject in one department for the duration of the course (so, if you are doing your degree in, say English, all your classes will be in the English department unless you are offered the chance to take an elective in a different department) and usually the first year is set so that everyone studying first-year English will be required to take the same classes at the same time so that everyone reaches the same level (I didn't actually get to choose any of my classes until I was in my third year and studying abroad in the US!).
Also, the first year of a UK degree doesn't actually count towards the final degree result, but you have to pass it to continue on to the second year.
With how the UK system is set up, could I take some summer courses to "get ahead?"
To be honest, I have never heard of any UK universities running summer courses (there is no 'summer semester' in the UK) - so I would guess that this is not possible.
I have taken a few courses in the US. I took a couple of college credits during high school, graduated from high school in Spring 2008, attended community college Fall 2008, and am currently attending university for Spring 2009. I don't think I have taken enough "major-related courses" to start in my second year in a UK university or even to get ahead by a semester, but with the few that I have, could the credits transfer?
Unfortunately, it doesn't usually work this way - you start the degree from semester 1, year 1 and study for 3 years and it is not easy to transfer credits between universities.
Also, because of the differences in the UK and US systems, freshman year of US college is generally equivalent to final year of UK high school - so chances are the classes you are taking in the US right now are boosting you up so that you will be ready to start your first year in the UK. When I was at university (UK), we had a US exchange student come over for a year - he was a junior at college in the US, but was put into the second year UK classes. And when I spent my third year studying abroad in the US, I was put into senior- and graduate-level classes.
So far all of the universities that I have applied to don't allow you start in Spring, but in September.
You can't start in the spring because the degree course runs from September and if you start in the Spring you will have missed out on the September classes and won't be able make them up (I don't know of any UK universities that allow a spring start). Also, in many subjects a class will run over two semesters, starting in September and ending in May with a final exam in May, so because of this, you cannot start your degree partway through the class. Also, you have to register in September for the entire year (Sept-May/June) - it is not possible to take a semester off here and there like you can in the US.
What is the difference between a module (UK) and a class (US)?
Essentially they are the same thing, just different terminology.
US class = UK lecture (for the actual class you attend each week),
US class (whole semester course) = UK module or course
How many modules do I take a semester?
That depends on the degree course - English majors may take three or four 2- or 3-hour classes/seminars/reading groups per semester, Physics majors may take six 50-minute classes/several hours of lab/1 hour of tutorial/3 hours of problems class etc..
