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).
ADSL2 (ITU G.992.3 and G.992.4) adds new features and functionality targeted at improving performance and interoperability and adds support for new applications and services. Among the changes are improvements in ADSL’s data rate, an increase in the distance ADSL can reach from the local telephone exchange, dynamic data rate adaptation, better resistance to noise, diagnostics, and a stand-by mode to save power. ADSL2 also reduces the initialisation time from more than 10 seconds (as is required for ADSL) to less than 3 seconds. ADSL2 has the same signal footprint as ADSL.
ADSL2+ (ITU G.992.5) doubles the bandwidth used for downstream data transmission, effectively doubling the maximum downstream data rates, and achieving rates of 20 Mbps on telephone lines as long at 5,000 feet. ADSL2+ solutions will interoperate with ADSL and ADSL2, as well as with ADSL2+. ADSL2+ will include all the feature and performance benefits of ADSL2 while maintaining the capability to interoperate with legacy ADSL equipment.