Acronym for Asymetric Digital Subscriber Line.
A technology that uses the frequencies above human hearing to send data at high speed to the exchange (a technique also known as FrequencyDivisionMultiplexing, or BroadBand). It's limited in range, the closer you are the faster the top speed you can get, up to 8mbit downstream. (The maximum distance is dictated by the Interleaving). It's Asymetric because the upstream and downstream rates are different, upstream being much lower than downstream. Much of this is academic however, as most ADSL providers have capped your bandwidth anyway although TelecomNZ are changing this. You'll need a special ADSL modem to use ADSL, and need special equipment at the exchange.
Because TelecomNZ owns most of the LocalLoop in NewZealand, you can only get consumer ADSL that is provided by them and resold. They used to call it JetStream but now call it Xtra BroadBand. Customers on TelstraClear's network can get a product that used to be called Tempest but is now imaginatively called High Speed Internet. This shouldn't be confused with the one they resell which they call PDQ (Pretty Damn Quick) BroadBand.