Ranching is just seeing how many of a given card you can collect, like you do with knights (incidentally, I have literally hundreds of various knight cards to spare, if you're interested).
My favorite card to "ranch" is of course Pestilence. I quit counting at 1,000, and I have them in every Edition ever printed, including some foreign ones. I was extremely pissed off that Pestilence was removed from 7th Ed., denying me the chance for a FOIL Pestilence.
I also ranch Earthbinds and Killer Whales. I was ranching Thrashing Wumpi for a while, offering $3 for every one on ebay, but that got expensive very quickly. Fortunately, it turned out to be a popular card, and due to a local shortage of the things (thanks to me), I was able to sell about 40 of them to a store and make a little money.
There is an added benefit to ranching. You know all those piles of cards we all have that we just can't rid of? Well, if everyone picked a couple of cards to ranch, and traded aggressively for them, they would all disappear, or at least be reduced somewhat.
Happy ranching!