DSL is a huge improvement over ISDN, because it is both faster and less expensive, but it is not available everywhere. Many people will find they can get ISDN, but not DSL.
You can get bandwidth from about 256KB to 1.5 MB over DSL. Typically, the bandwidth coming to you is much larger than bandwidth going out from you. (The "A" in ADSL stands for Assymetric.) That's normally just fine, because all you generate is requests for pages. Most of the traffic is the response coming back. The phone companies encourage this assymetry, so that people don't start putting a lot of servers at the end of inexpensive DSL lines. There are a number of minor variants of DSL, generally proprietary to a particular phone company. So the general term "xDSL" or just "DSL" is usually favored.
DSL allows you to use the phone line as a regular voice phone line at the same time it is being used for a data connection. So if you order a DSL line, it may be the only line you need.
If you get DSL, you will also get a piece of equipment at your end, which you can get from the telco or buy yourself. It is a DSL router. (It may be called a "modem" but it isn't, really.) They generally come in two types: one is really a modem replacement: it supports one machine connecting to the DSL line. It will probably use an Ethernet connection because that is faster than using a serial line, but it is not designed to be used on a network.
The other kind of router you can get is designed to connect a LAN to the ISP. This router will connect directly to your ethernet and, given that you have the IP set up right, all the machines on your network can use the DSL line simultaneously. See Routers for more detailed discussion. The Router discussion is applicable whether you are using DLS or ISDN or any other kind of connection beyond a simple modem.
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