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The quality of your ISP will be a big factor in how good your DSL performance is, how reliable it is, and how much technical support you can get when you have problems.
You are free to use an ISP as just an on-ramp to the internet, (including ignoring any email facilities they would provide for free, or their usenet news servers, or their home page disk space) but these services are available should you need them.
If you get ADSL from the phone company, you may find they strongly suggest using their own ISP (for example, Pacbell has Pacbell.net, Bell Atlantic has BA.net), and that may be the simplest choice for you.
However, with the large number of internet 'service companies' providing everything from mail (hotmail etc), DNS (easydns.com etc), web hosting, news (supernews, dejanews etc), it is possible to be almost totally independent from the ISP, once your connection is up and running.
Apart from the Telco, the companies building large DSL infrastructures do not want to deal with residential customers directly, recognizing that this is often expensive and not their forte. So what they do is sell their lines to traditional ISPs, who now offer DSL, or to newer DSL-only ISPs, who are more marketing oriented.
The ISP provides your gateway to the Internet. When data leaves your house, it is not immediately on the internet. Instead it travels to your ISP first, and then to their internet gateways (known as upstream providers). The ISP provides DNS service for your static IP address, if you were a small business, or handle allocation of dynamic IP addresses (DHCP) otherwise. They would also provide you with email, a usenet server and more IPs if you need them. They are also the first point of contact for installation, billing and technical troubleshooting. Pick a good one! (which is what this site is all about).
Also read About DSL for lots more information