What is the difference between a radian brake lever and a normal brake lever in a motorcycle?

Answer

I'm guessing you mean "radial" brake levers and not the '80's Yamaha 600cc bike Radian?
If so, the premise behind them are greater control, feel and accuracy of your breaking. Greater force applied to the rotors by the pads with less force from your hand. They also cost anywhere from $100 and up MORE THAN a regular lever.
It's very easy to tell them apart visually so I needn't go into that here.
If it's braking ability you're after with your bike I would suggest quality pads and lots of practice first then when you're ready for the speeds required to get the benefit of a radial mount brake set up look into it then.

Answer

From Todd Robinson (Cal-Sportbike and the TrackXperience riding school)

The difference is not in the brake lever per se, rather it's a design of a master cylinder, and the lever is integrated into the unit. Note the pic below, where the plunger goes directly into the piston (usually 16-19mm bore) and connects to the lever about 18-20mm from the pivot point. As stated above, a radial master cylinder is not the first modification that should be done. The biggest night-and-day difference is braided stainless lines coupled with good HH (sintered) brake pads and high temp (like Motul 5.1) brake fluid. But the next step, before calipers and rotors (in most cases) should be a radial master cylinder. We suggest 19x18 for most sportbikes with 4 pistons, 19x20 for 6-piston calipers, and 16x18 for Ducati or single caliper applications.

http://www.calsportbike.com/images/productbrembo_mc.jpg

• How do you tell if a tire is tube or tubeless?
• Can anybody explain a motorcycle and all the parts and how they work because you are a beginner?
• Will suzuki rm 125 graphics fit on a suzuki drz 125?