Your school will determine what portion of the total will be subsidized and what will be unsubsidized, based on your calculated financial need. So, while you'll be eligible for $7,500 (assuming it does not exceed your school's Cost of Attendance budget), we can't tell how much would be sub and how much unsub because we don't know what your financial need is.
If you're being offered $7,500 I'm going to assume that you are a dependent junior or senior. If that's the case, the max you could receive as a subsidized loan would be $5,500 and the rest would be unsubsidized. However, if your need is less than $5,500, then you would receive less sub and more unsub.
With a subsidized loan, there is no interest while you are in school. However, interest is charged once you cease to be enrolled, either because you graduate, or because you withdraw.
With an unsubsidized loan, interest begins to accrue as soon as the loan is disbursed. This means that you will be charged interest while you are in school, even though you may have opted to defer the actual payments until after you leave. If you choose to defer, then the accumulated interest will be added to the principal of the loan when you enter repayment, so you will actually pay back more than you borrowed. In effect, you will pay interest on the interest. You can avoid this by paying the interest while you are in school (it's usually only a few dollars per month). You can also choose to begin repaying the loan while in school, and that can also save you quite a bit of interest.