AH64:
ADSL stands for Asymmetric Digital Subscriber Line
In english that means using different data rates for data send out from your PC (Upstream) and data received by your PC (Downstream). The rationale behind this is that you receive more data than you send - therefore it is more efficient use of the total bandwidth.
The 512k quoted is the speed of the downstream leg.
To expand on rustle's answer:
512kbps = 0.512mbps
Hope this helps