As I said earlier 80 is used for HTTP, 21 for FTP other applications prefer certain ports secure FTP uses 22.
I am not sure if there is some way that the pc blocks applications or entries that try and use the same port as another application, or if its restricted soley to that one, I assume it must work like that or we would only have a single port for all traffic to and from a pc.
If an application needs to use a specific port them it usually says so when being installed or if, as Simon says, its a P2P program then you can usually select which ports you want it to use so you can restrict the likelyhood of a hacker using known sharing ports.
Most routers should block ports by default, apart from the ones mentioned above, others have to be added either by using the applications/games menu, port forwarding, or creating virtual servers.
Different routers have different terms for the same thing.
Also some routers need ports configuring for TCP and UDP, others can do both in the same rule thats been created.