For CLEC provided DSL (ie, DSL through Covad, or Northpoint), It is the process of taking the line from the drop, where the telco left it, to a socket on your wall. This may involve running new lines at your premise, but this process is no more painful than adding another phone extension. For ADSL, inside wiring is handled by the Telco as part of the whole install deal. Where the Telco and the CLEC must cooperate with wiring, there can sometimes be extensive delays and difficulties, none very technical, usually a lack of coordination.
For older apartment buildings, there may be much bigger problems, if there are no spare pairs available for non-Telco DSL, new wires may have to be run in the building, adding to the expense, time delays and trouble.