ADSL: Short for asymmetric digital subscriber line, a new technology that allows more data to be sent over existing copper telephone lines. The old Dial up internet also used the phone line but used the same frequency, therefore you could not use the phone while surfing the internet. ADSL operates on a different frequency and you can use the internet at the same time. Your phone may require a simple filter to remove any white noise that can be caused by using both.
ADSL internet connection is always on, whereas with dialup internet you had to connect and sign in each time. In order to use ADSL you will need an internet service provider and a ADSL modem. With technology developing at a rapid pace, the speeds available on ADSL are increasing every few months.
An ADSL modem is the device you have to use to connect a computer or router to a telephone line in order to recieve the ADSL connection through from the telephone line.