A good way of describing this is an analogy to a motorway. 70mph is your maximum speed limit on the motorway and is equivalent to your line speed. As we all know too well this doesn't mean you can do 70mph all the time, especially if it's been snowing or the road's choc-a-block with other traffic. The speed you're actually able to do on the motorway is equivalent to your throughput speed.
Line speed is the maximum speed that your line is capable of supporting. Your line speed will vary depending on a number of factors such as line quality, how far away your exchange or cabinet is, and whether the ADSL micro-filters are installed correctly.
Throughput speed or download speed, is your actual speed transferring data over your broadband service. Throughput speed can be affected by the individual speeds of websites you visit, the number of other people currently using that website, and whether you're accessing at peak times (similar to rush hour on the roads).