Below is a great article from Racecar Engineering which compares Mercedes log exhaust versus Ferrari's tubular exhaust. Much has been made about the benefits or not of the Mercedes log exhaust and this article looks in depth at the benefits of each. Right click>view image for a more clear image.
It's a funny term. It is using steps - sharp variations in diamter, using pulse chembers, X-pipes, H-pipes or whatever means necessary to even out the pulses from the engine to be smoother and to give less pulse reversals. Since the turbine wheel is always loaded by the compressor any drop in the pressure delivery means a quick deceleration of the blades. Then to get the blade back up to speed you have to fight against that inertia. It makes boost generation slow and inneficient.
Think of an AC rectifier with a smoothing capacitor and you get the idea.
I do not trust that article too much though. i know that tubular manifolds can give massive inscreases in turbo spooling. Though for a given design rpm range. Logs just kill all the pulses.. Mercedes must have some fancy chambers inside their log to keep any hope of superimposes the pulses from the engine. I rather think the only advantage they have is heat retention and more stable pressures. I don't think pulse tuning is the point of the turbo log. haha.
http://www.f1technical.net/development/461