I have some questions about the roles of ISPs, DNS, how packets work, etc.

For example: What service do ISPs actually provide?

I'm trying to categorically distinguish between transit ISPs and regular ISPs, Tier 1 networks, what an "IP backbone" is, what factors determine the "speed" of the Internet outside cable type, length, and an ISP's arbitrary rate-limiting as per their consumer offerings (and not merely the speed that users receive, but the theoretical maximum speed).

I looked at the FAQ for the various computer/programming-based SE sites (SO, Programmers, IT Security, Server Fault, Theoretical CS, etc.) and this place seems the most appropriate, but I just wanted to make sure.