You might not realize it but you're using an Internet Service Provider right now. ISPs are organizations that provide connections to part of the Internet. If you're connected to TILT from your university, then your university is acting as the ISP and might even be charging you a small fee to use its services.
In the outside world there are literally thousands of service providers -- from the big guys like AOL to smaller local providers. While they will all allow you access to the Internet, the key word here is "service." Some providers will give you a lot of help in answering your questions, fixing things quickly when they break, and generally assist you in having a pleasing Internet experience ... others might not be so helpful. Shop around when looking for a provider.