The ACL is the really strong ligament that attaches your top leg bone to your bottom leg bone behind the knee cap. If that is gone, you lose most of your stabilization in your knee, meaning that if you try to cut or something, your knee could go in any direction.
The other ligaments in the knee sort of wrap the joint in various ways, so that if any of them are strained or torn you have some discomfort and lose some mobility, but you are not nearly as "wobbly" as when your ACL is gone.
ACL surgery can be performed orthoscopically, and usually the repair is stronger than the original. They just screw in a new ligament, taken either from a cadaver or grafted from another knee ligament. The rehab isn't too bad, if it's done right. The problem is they sometimes get in there and find more wrong. And the longer you go without getting it fixed, the greater the likelihood that you will do additional damage to the cartilage and ligaments in your knee.
Tough call on playing with a brace. They make some pretty darn good knee braces, and I have heard of football linemen playing with all kinds of knee damage with a good brace. How well they adapt to roundball, though, I don't know.
Brian96--ACL patient in 1995 and 1999