P2P Network and uploading incomplete files

[Trynty]

Hi I've been lurking around this forum for a couple days now trying to learn about P2P sharing since I'm new to this whole thing and I think I've figured a lot of it out.

There is one thing that I've been wondering about though...Right now I'm in the process of downloading a movie which is of course huge. I figure it will take me a couple days for it to complete which is perfectly fine with me. The thing that I have noticed though is that there are several ppl that are uploading the same movie from me that I am downloading from someone else.
I'm a little confused how that is possible since I don't even have the completed file yet.

I have a couple concerns 1.)How do I know that the file I'm dwnlding is complete 2.)Will the fact that ppl are uplding an incomplete file from me affect their dwnld

In case you were wondering I have a DSL connection and within the last few days when I connect I've been an Ultrapeer. Does that have something to do with it?

Mostly I'm asking the questions in an endevor to educate myself and learn more about how P2P, Limewire in particular, works.

Thanks so much gang for any info you can send my way

PS If this isn't the right thread for this question plz direct me to the correct one. As I said I've been lurking for a while and this ques doesn't seem to fit anywhere else seeing as how it's a genral question and not a problem persay. But if I'm mistaken I'd be more than happy to repost it if neccesary