The "C" model Glock pistols
versions of the very same model. The Glock 17C
is basically a Glock 17 with a ported barrel
and a ported slide
The porting on the barrel of the Glock 17C are two elongated gas ports that are positioned atop the barrel, on both sides of what would be the "dorsal" section of the barrel, a few millimeters before the muzzle. The slide also has two bigger ports on top of the barrel ports that allow for the gas escaping from the barrel ports to exit.
The idea behind porting or compensating is to reduce muzzle flip or muzzle jump by redirecting the gases generated by the explosion of the propellant upwards almost simultaneously with the bullet's exiting of the muzzle. This greatly reduces perceived recoil and makes for faster reacquisition of the sight picture for quicker follow up shots.
The downside is that since gases are vented a few microseconds before the bullet exits the barrel, the gas pressures are significantly lower which results in lower velocities. The ports also create much more muzzle flash which makes a ported pistol not very ideal for a self defense sidearm that might have to be used in low light conditions.