Please Help! Buying First Bong, So Very Confused

Hey All,

Long time no post, but im back, i think. I have always been a big toker of joints and when im not smoking a j im smoking out of my spoon pipe. I do have a small glass bong but its really just a mini hitter so ive never thought about new parts. However im going to be buying a Blaze Bong and have become really confused regarding all the terms and measurements. Can anyone explain, in plain terms what the joint size is, what the ground size is, what a diffuser is and what are male/female joints?

Here are the bongs specs:
Joint Size: 18.8mm

Reducing Diffuser: 18.8mm - 14.5mm

Bowl Size: 14.5mm

and here is the downstem i hope to buys specs:
Male Joint: 18.8mm

Female Joint: 14.5mm

would that downstem fit? would i need to remove the diffuser and if so is there any benefit to a diffuser? what bowl would fit the current bong and what bowl would fit the downstem?

in my 3 years of smoking i hate to say im a bong set up newbie and really need some help.

Thanks in advance :D