Just curious, what are UDP ports and why do they need to be open as well as TCP?

Also, there are a few posts complaining about file corruptions and someone commented "well, the public think they work" - well they do!!! Should I be worried that 25% of my "short films" downloaded with warnings but seem to play ok? Am I spreading them to other people who can't play them for some reason?

Can anyone explain what type of file corruption it detects and could it be actually caused at this end as sometimes my computer crashes during a download?