If you have a p2p application such as limewire. and if you have a shared folder designated as such, in which you will have files you wish to share, then any other user with an application like limewire, connected to the Gnutella network, can view what you have in that folder, or find a certain item in the folder by making a suitable search, and can download a copy of that file. They do not ask permission they just do it. that is what you do when you download a file from someone else.
They cannot view anything else on your computer or download anything else, only what you are sharing.

I hope that makes sense, please ask if it doesn't.

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