ADSL stands for Asymmetric Digital Subscriber Line service. It is called asymmetric because the download and upload speeds are not symmetrical (download is faster than upload).
ADSL broadband comes entirely through the copper broadband / telephone network, which isn't hugely efficient. Advertised speeds in New Zealand generally hover between 'up to 24Mbps', but you'll likely see an average of about 10Mbps download and 1Mbps upload speed. This is plenty for emailing, surfing the web and checking social media. But remember, the copper network was designed for carrying our voices back and forth, not for you to stream the latest episode of Game of Thrones.
Technically speaking there are two types of ADSL Broadband services within New Zealand but these are now generally grouped simply as ADSL. The more modern tyoe of ADSL is known as ADSL2+ whilst the original is just called ADSL.
ADSL is the most basic broadband connection available on the copper network. It delivers around 2Mbps for distances up to 6km from the local cabinet. ADSL is mostly found in rural areas where new cabinets are yet to be installed.
ADSL2+ delivers speeds of up to 10Mbps to properties located within a 2km radius from a broadband cabinet. It delivers faster speeds but its performance can be influenced by a number of factors like how far away you are from the cabinet, the devices you are using, the quality and age of your modem and your home wiring.