Good answer from gofastman. I'll just add the following.
PG is thin, almost like water.
VG is thick like a cold corn syrup
Higher percentages of VG sometimes cause your wicks to run a little dryer when vaping, meaning you might get a few more 'scratchy' or dry hits.
Higher percentages of PG can sometimes mean you get too much liquid into the coil meaning you might get some gurgling or liquid in your air paths of your device.
Almost all ratios are expressed in PG/VG so an 80/20 will be 80% PG and 20% VG.
There is no right or wrong ratio though. If you are looking for throat-hit, go with maybe an 80/20 or a 65/35 and if you want more clouds, then try a 20/80 mix or something.
I have 2 favorite juices, one is a 100VG and the other I mix down to about 40/60 or 50/50, but you will find your own sweet spot.