It can be confusing because they both do something very similar. I wish there was a more clear explanation with road conditions and speeds.
Here is what Volkswagen's technical glossary says:
Traction Control (ASR)
For the high-torque engines, the traction control system offers more comfort and safety, particularly when driving on slippery roads or surfaces with differing degrees of grip. The traction control system facilitates smooth starting and acceleration over the entire speed range without wheelspin or fishtailing.
The traction control system operates in tandem with the electronic accelerator (E-Gas) and uses the wheel speed sensors of the anti-lock braking system (ABS). If a sudden increase in speed is detected at one of the drive wheels (slip), the traction control intervenes in the engine management system to counter this effect by throttling back engine power.
Traction control assures good traction and directional stability in the acceleration phase across the entire speed range and thus promotes active safety. In addition, it reduces tyre wear. Traction control also incorporates the electronic differential lock (EDL) and makes up part of the electronic stabilisation program (ESP).
Here is what they explain for EDL
Electronic Differential Lock (EDL)
The electronic differential lock permits smooth, comfortable starts on split-friction road surfaces with differing levels of grip. If one wheel starts to spin, the electronic differential lock will brake the wheel as necessary, directing power to the wheel with better grip in the process. The electronic differential lock reduces tyre wear and operates at speeds of up to around 40 km/h (4MOTION: up to about 80 km/h). As a software function, it forms part of the electronic stability control (ESC) and traction control (ASR).