The file will be delivered as an Illustrator 8 EPS file. Most vector-based software can open and edit an EPS.
Once you've purchased the file, one person (probably you) can use it for almost anything. You're just not allowed to re-sell the file in any way. If you want to sell products that use the vector (t-shirts or posters or whatever), get in contact for permission. I'll probably say yes.
You don't have to credit me, but I'd really appreciate it.