看没人答你，我就花点时间给你说清楚。既然你在美国，我就用英语了。
DSL is digital subscriber line, which uses the high frequency bandwidth of the telephone line to carry the digital signal of the internet。
    There are quite a few DSL companies and different bandwidth depending on the price。  Comcast cable uses the coaxial cable to carry the signal。
    The difference between DSL and cable is where the signal gets pooled。  For DSL, you have your own, dedicated line until the central office, where all the signals go out to the web。
    For cable, you have shared bandwidth from your house。  Depending where you are in the states, cable and DSL are highly variable when it comes to quality。
    For the same price, cable is usually faster。  But speed may vary during peak hours if Comcast doesn't care about your area and didn't add extra bandwidth until many people complain about it。
  
SBC owns all the telephone lines in california and some other states。  Due to the line share requirement established by the FCC, it has to let some companies to use their lines for a fee。
    That's why you can get DSL through many other companies besides SBC。  If you are in California, I highly recommend do NOT use SBC DSL。
    They have very bad service。