Kasseri is a Greek cheese traditionally made from sheep's or goat's milk, though inferior cow's milk versions exist (one in particular made by Bel Gioiso).
Kasseri is a mild, firm-textured, slightly crumbly cheese (softer than parmesan but firmer than, say, gouda), with a color somewhere in the range of off-white to golden. In my experience, the more yellow the kasseri, the better its flavor. It has a a fresh, somewhat sweet taste and a hint of sharp tanginess. The tang comes from the brief aging process--about half a year of aging depending on the manufacturer.
Kasseri is quite delicious; it pairs well with olive and lemon flavors, and it is also a terrific general-purpose cheese. It melts relatively smoothly and goes great on a sandwich. It is wonderful with fruit or preserves, such as quince preserves or guava paste. A satisfying breakfast can be had by lightly toasting a piece of bread, spreading the bread with apricot preserves, and laying a slice of kasseri atop it.
Kasseri is also the traditional main ingredient in the incomparable Greek dish saganaki, though I prefer kefalotiri in this dish.
I should add that the best place to get Kasseri, besides in Greece or on a Greek Island, is your local Middle Eastern Import Food Store, or dedicated cheese shop. If possible, avoid buying kasseri (or any other specialty cheese) at the supermarket, it just won't be as good.