[Merkavaboy]

what caliber exactly does a glock 19 use because I'm confused ?

When people talk about the modern 9mm caliber pistol, they are referring to the pistol caliber & cartridge known as:
9x19mm (9mm or .355" caliber bullet with a case lenght of 19mm)
9mmx19 (as often referred to by the British)
9mm Para
9mm Parabellum
9mm Luger
9mm Luger +P
9mm +P+
9mm NATO (NATO symbol that's often seen on boxes and the cartridge headstamp is a circle with a cross or "x" in it)

Those are the most common terminology that you'll see on boxes of 9mm ammunition.

Is there the option of 9mm but with different lengths or something ?

There have been a plethora of different cartridges that were named "9mm" and there are other cartridges that are currently in use today that have "9mm" in their names, such as:

9mm Kurz/Corto (which is the .380 AUTO/ACP cartridge)
9mm Short (.380AUTO/ACP)
9x17mm (.380AUTO/ACP, which has the same .355" dia. bullet with a 17mm case length)
9mm Makarov/9x18 Mak (USSR designed pistol cartridge with a .363" dia. bullet and 18mm case length)
9x21mm IMI (9mm/.355" dia. bullet with a 21mm case length)
And on and on.

All that you need to know is that NONE of the above cartridges can be chambered and/or safely fired in a pistol that is chambered for the 9x19mm cartridge.

Also for a beginner at a range learning the ins and out of the sport and target shooting what is a good round for a G19?

There are many brands and types of ammunition available on the commercial market right now, including full metal jacket (FMJ) practice ammo and modern self-defense jacketed hollow points (JHP) by Remington, Federal, Winchester, Speer, CCI, Sellier & Bellot, WOLF, Fiocchi, Magtech, etc.

For general practice, you can use any factory loaded 9x19mm ammo you want, but for economy, the FMJ ammo is often the cheapest to get for practice and plinking. Use modern hollow point ammo from the major U.S. manufacturers for self-defense.

The use of reloaded/remanufactured ammo voids your GLOCK's warranty if something happens while using said ammo. RUN, DON'T WALK AWAY from reloaded ammo being sold by people at the local gun shows or at the local range when it comes in little plastic baggies, because you will never know who made the ammo and whether or not they are bonded/insured for product liability.

But again and please excuse the long windedness what exactly does a G19 use - round wise ??

The 9mm GLOCK pistols were designed to shoot any factory loaded 9x19mm ammo, as long as they are jacketed so that the metal of the bullet jacket is in contact with the bore of the barrel. Exposed all-lead bullets are a no-no. Small rings of lead from the exposed bullet can be shaved off and build up and cause excessive chamber pressure that can lead to a catistrophic break down of the pistol (in otherwords, kaBoom! [kB!] ). Thus, GLOCKs can shoot any factory loaded 9x19mm ammo that's produced to standard pressures, +P pressures, +P+ pressures and NATO pressures. Ammo made to +P, +P+ and NATO pressures will usually say so on the box and/or on the headstamp of the cartridge for easy identification of higher than standard pressure ammo.