What is a full-time student?
For Undergraduate students, "Full Time" means that you are taking 12 or more credit hours.
For Graduate students, "Full Time" really depends on the students situation as well as the department they are in. First, contact your department and ask what you need to be "Full Time". Also give the policies website through Graduate Studies a look.
This is generally important for your health and car insurance (you usually get discounted rates when you're a student), and for scholarships & loans (you generally don't get them when you're not full time).