ADSL2 are new ADSL standards that enchance existing adsl standards and utlize newer technologies. ADSL2 can run up to 12MB/s (regular adsl is 8) and can give another 600 feet in distance from the local exchange. 2+ goes even further, bringing the total speed to 25MB/s and has about a 1.5 KM range from the local exchange.
They basically are able to enhance speed and distance by changing the algorithms that are used to interperet frequencies (how the data is transmitted over the phone line). 2+ also has the ability to multiplex lines, which means you can combine phone lines and get another 25MB/s each line you add.
Hope this helps...