What's the difference between a Subscriber and a Contact?

I'm brand new here. I appreciate any help you can give. I uploaded 9 .xls spreadsheets of contacts that I received OUTSIDE of Constant Contact. On my account page called "Contacts," I see all 9 lists and the number of "Contacts" in each one. The sum of all of them = 1367. It took a lot of work to GAIN those people's interest AND to also enter it into the spreadsheet. The "Contacts" page indicates that I have 890 "Subscribed" - I really don't understand. Where did the other 447 people's emails go? When I export a spreadsheet, I have 890 people in the export... but I just don't understand why I don't have 1367. I highly doubt there were over 400 overlaps/ duplicates. I'm really struggling to understand what happened. Any help would be greatly appreciated, DJ