What is the difference between ADSL and SDSL?

ADSL - Asymmetric Digital Subscriber Line basically means different upload to down load speed.
SDSL - Symmetric Digital Subscriber Line means the same upload speed as download.

For further info here's a couple of links ADSL (http://en.wikipedia.org/wiki/ADSL) & SDSL (http://en.wikipedia.org/wiki/Symmetric_Digital_Subscriber_Line)