By strict definition, a college is a part of a university. A college devotes itself to one field of study. For example, I could say I have a degree in mathematics from the college of mathematics at the university of A. University A can be made up of many colleges, each with its own field of study and sets of degrees. In actual usage, however, especially in the U.S., people will talk of a college and university as if they were the same thing. It is only when we talk strictly about how a university is organized that we get into the fine differences between the two terms.

An University has the rights to set a syllabus and formulate courses... A college is usually affliated to a certain university and follows the courses and syllabi set by the university :) An university can have any number of colleges under it :)

I have just one question. how should high school students attend the community college? should they take any special exam ? I wasn't sure how to comment on the question, so doing it on the main question... Typically students are told by a teacher that a college class is available for 'dual-enrollment.' This is the term used in high schools when you're a high school student, but you're also taking a community college class. All you have to do is follow the registration process as followed by your high school. There is no exam needed to take the classes; however, sometimes teacher recommendation/approval is needed but the majority of students who choose to take these classes are already serious about their studies so the approval is really just a formality; but for some classes you just have to show proof that you're a high school student and that you're a resident of the state where the college is. Obviously, there are thousands of high-schools in the USA so each one might have a different requirement; thus I'm speaking based on my own experience and that of friends who took "dual-enrollment" classes when they were in high school. If you want even more specific information, google "dual enrollment" and the name of a community college. If you don't know any name, just type any usa city followed by community college. For example, miami community college dual enrollment or denver community college dual enrollment, and on and on....

To add to what fdmaxey said, in the USA you have places known as community colleges which are also known as junior colleges. These institutions provide a two-year higher education degree/diploma or certificate or and associate's degrees. Many also offer continuing and adult education. When you're in high school, you have the option to take classes at the community college, and these classes could then then transfer to a university. For instance, you could take a chemistry class in the community college and when you finish and pass the course, you get a certificate which you can then present at your chosen university, and most universities will credit you for this class. Also, you can get an associates degree in vocations like nursing, healthcare technology, car mechanics and more. Typically students will attend a community college to save money on university costs. For example, you can take a class at the community college for 100 USD, and the same class at a university might cost you at least 600 USD. Or sometimes you want to get a vocation that won't take 4 years to complete and many community colleges offer these. Many people will argue that classes at the community college are easier and therefore it is an injustice to allow people from these colleges into programs at universities, but obviously that all depends on who your teacher was. I took classes in community colleges which were as difficult as those offered in a university, and the instructors offered real life examples which made the class a lot better, while I also had university professors who couldn't teach a subject to save their lives! :)