Magic The gathering - what is a set of them?

You can tell if you have rares if the symbol right at the lower right hand corner of the picture is gold.Your cards are most likely older so I doubt they'll have the colored symbols.From what I know old cards don't sell for much unless they are still in print,or are incredibly good/rare,but considering you have "10,000" cards you can probably make a lot.I'm not sure what constitutes a set.

Well...it's unlikely that you have a full "set" of anything. The whole of Magic is well over 10,000 cards (I don't remember the exact number, but I know it's much higher), and I also doubt that you have even 1 of each card of a specific set.

To find out if a card is a rare, you either have to look it up in the Gatherer ( http://ww2.wizards.com/gatherer/ ) or look at the expansion symbol of newer (Post-Tempest Block) cards. Newer cards have colored expansion symbols; gold is rare, silver is uncommon, and black is common. Older cards will just have a black expansion symbol for everything.

The expansion symbol is about halfway down the card on the right, just below the picture and to the right of the card type box.

To be honest, it probably isn't worth a whole lot. There's a very few select cards among the old cards that are worth anything, and most of them were extremely popular cards back when they were new as well. Since most cards are common (and old commons aren't worth much - generally about five cents to fifteen cents), calculate the value of your collection at about thirty cents a card (and that's a generous estimate). So, approximately $300...and that's if you can find a buyer willing to just buy a large collection likely to be junk.

However...if you can find certain cards among your collection that are still used today, you could have some cards worth quite a bit. Some of these cards are Force of Will, Time Vault, the Moxen, Black Lotus, Library of Alexandria, Time Warp, Ancestral Recall, and any of the dual lands from Alpha/Beta/Unlimited/Revised.

Of course, there are other valuable cards; those are just some examples. If you want to know that value of a card, go to starcitygames.com and look it up.