Because the meniscus is a big piece of cartilage basically, it can tear in all different ways. Some people just have the torn piece removed and smoothed off, in hopes that there is still enough there to cushion the knee sufficiently. If the tear is small and clean enough however they can stitch it back together and wait for it to heal properly. This takes a ridiculous amount of time given the lack of blood flow to the meniscus.
It's not a tendon as I stated above and others in the thread have. The meniscus is fairly large and is more of a cushion than a stabilizer. To be perfectly honest, having a clean ACL tear is probably better than having a meniscus tear (unfortunately they usually tear together because the knee buckles and gets torn up.) The ligaments have better blood suppl and can be stretched back out. Once meniscus is removed that's it, it's gone forever, similar to cartilage.
People keep bringing up B. Roy, but IMO the better example is Chris Paul. he's had a piece of his meniscus removed, and while he's still a great player, he doesn't have the same explosiveness and numbers he was putting up before. From everyone I've talked to who has had the procedure done, you're never quite the same.
edit- When I say the procedure I mean removing part of the meniscus. A meniscus repair is less common, given the recovery time people opt out, and on top of that it requires a particular type of tear for it to be repairable.
