Sorry, but that's still too abstract for me. Let's say I'm a college kid, and I decide to upload a picture from the party last night. How do people know that

Sorry, but that's still too abstract for me. Let's say I'm a college kid, and I decide to upload a picture from the party last night. How do people know that the file is there? Do I get a URL, that I give to them? OK, now the file is uploaded. Will it stay there forever, or is this just temporary? Let's say some people want to download it. The download page is a chatroom, but what are the odds that more than one person would be downloading at a time? Do you have any metrics from other file-sharing sites about this? I'm still not picturing who this is aimed at.