In general, and there are exceptions, there are two academic "calendars" in use in the US; the Quarter and the Semester.
The Quarter system: The year is divided into 4 quarters; Fall, Winter, Spring, and Summer. A "traditional" full time student schedule is 3 to 5 classes for each of three quarters with Summer being taken off. Some students take classes in the Summer to graduate sooner.
The "hours" taken refer to actual time spent in class each week. A Western Civilization course that meets 3 times a week for about one hour each meetng is typically a 3 credit hour course. General Chemistry that meets 3 times for lecture and has a three hour lab typically counts as 4 quarter hours.A Calculus course that meets 5 times a week for about an hour is counted as 5 quarter hours. Thus if you took Western Civ, Chemistry and Calculus you would be taking 12 credit hours. This is a light but full time course load most places.
Many institutions offer longer but less frequent class sessions to facilitate scheduling for working students. A five credit hour course might meet only twice a week for 2 or so hours each meeting. The assumption is that less frequent, but longer classes are more efficient due to reduced administrative overhead during class.
The Semester: There are two of these in a traditional school year; Fall and Spring. There may be special summer sessions at semester system institutions.
The credit hours are still based on the number of hours a week you attend class. The difference is that you attend a course for more weeks. Thus a semester "hour" counts more toward graduation than a quarter "hour". Generally 8 Semester hours equal 12 quarter hours.
Typical Example; A full year of General Chemistry (needed for Med School!) requires 3 hours of lecture a week and a 3 hour lab (no bonus hours for danger, lab hours count less than lecture hours). In the semester system this is divided into two 4 hour courses for a total of 8 semester hours. In the quarter system you take 3 four hour courses for a total of 12 quarter hours to cover the same amount of material. The 2 semester and 3 quarter courses are equivalent for graduation, application to graduate/professional school, etc.
As always, institution requirements are definitive. You do what your school says to graduate, just like in high school but with more choices (and fun!).
I hope this is helpful. My DD firmly believes there is a picture of me in the Oxford Unabridged Dictionary next to the word "pedantic".
Last edited by BigG; 02-28-2009 at 09:30 PM.