Basically backing up what geoff said, if you have cable, or even a cheapo DSL line, then you dont get a sustained line, you get subscribed to a DSL "pool", this pool will have say a 90Mb/s connection coming into it, theneveryone in that pool will share that conection, so if a few people are downloading or streaming then your conenction will suffer.
And incase your wondering now, yes its theoreticlaly possible to get a 90Mb/s connectio to your house if you were to locate and sabotage eveyone elses connections.... theoretically.
dragon2309