DSL is always on. This means that you are always connected to the internet. When you turn your computer on you log on, which means that you submit authentication information, much like logging on to a PC at work.
A dial-up is a connection where your modem dials a number to connect to the internet.
With always-on connections like DSL or cable, there is a greater danger that someone will decide to have a play with your IP connection (most of the time IPs for cable and DSL are static but mine isn't). Firewalls protect you from people who may want to toy with you (like Vaskez mentioned above).
www.zonelabs.com provides you with a free firewall to download. Or you can get Black Ice Defender, like I did.
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