Work studies are jobs you can have while your are a college student. More specifically, they are jobs that are offered to students as part of a financial aid package. If you are eligible for financial aid, a college may offer you a package that covers part or all of your tuition and room and board costs. That package may include scholarship money and work study as another. For example, you may be given a package that is $10,000 in total, which is made of $8,000 of scholarship money and $2,000 that you can earn through work study. That means, you will have to find one of the work study jobs available on campus (which in my experience, included things as simple as sorting books at the library and as interesting as a research position in a lab), and earn up to $2,000 which can be used to pay for your expenses.
Work study typically refers to one type of financial aid where you have an on-campus job and part/all of your earnings goes toward your tuition; it's a student contribution to the financial aid agreement. However, there are also plenty of jobs that you can get in college that are not work-study eligible.
Is there a particular kind of student job that you're especially interested in?