There are some knee guards and knee pads out there which claim to be braces. These are basically just beefed-up knee pads with hinges on them - here are a few things to look out for when shopping for a set of knee braces:
Poly-axial hinges: The hinge should have 2 pivot points for each side of the brace, if the brace has a single hinge point it will not pivot in the natural motion of your knee.
The hinge point: Set just behind the middle of the knee joint will provide the best protection.
Protection for the front of your knee: A solid patella cap to protect against impact trauma, particularly above the knee.
Long bars or arms of the brace: The longer the better really as this is what is anchored to your skeleton which gives your joint support.
Good strapping: Dual strapping is best, two straps above and two below keep the brace fixed to your leg and prevents it from sliding up and down your leg.
An adjustable extension locking point: The better braces will allow you to adjust how straight your leg can go before the brace reaches its lock out which prevents hyperextension.
Anchor for the bottom of the brace to the boot: Twisting the lower part of your leg can happen without even falling off the bike so this is a great preventative measure against it!