ADSL
The A in ADSL means asymmetric DSL. This recognizes that for home use, since people tend to be more of a consumer of data than a producer, a slower upstream (upload) speed can traded off for a faster downstream (download) speed. In addition, the ADSL standard allows for regular phone service to be squeezed into the low end of the frequency spectrum. ADSL therefore needs a splitter to be placed during installation.

IDSL
IDSL is DSL at 144 kbps. IDSL uses ISDN transmission coding, bundling together both ISDN channels and voice all on one circuit. IDSL does not use any kind of dial up nor involve per-call fees. For those that live too far for regular DSL, IDSL may be the only DSL option. IDSL tends to be priced at a rather higher rate per bit of speed, than any regular DSL. IDSL can still be a very satisfactory solution for data transmission compared to the alternatives (modem), coping very well with online gaming, and medium quality streaming audio/video.

SDSL
The S in SDSL stands for Single line DSL. SDSL needs just one twisted pair (line), whereas some other earlier DSL standards needed two, or even three pairs. In addition, it is useful to remember that this standard is symmetric, ie, the maximum data rate both upstream and downstream is the same.