Optic selection used to be much easier than it is today. Manufacturer X offered 1 reticle option and you worked around it.
Today there are still a few manufacturers who offer 1 system only, but this is changing.
The industry standard for what was considered precision shooting 35 years ago was Mildot (Mil). It was a system that originated with the military and like all systems has pros and cons. It is referred to as the milradian system as it encompasses several variations of reticle that employs the milradian unit of measure. The pro here is that it is divided into 1/10 increments for change so only very simple math skills are required. The con to the milradian system is the units of measure or incremental change are quite large compared to what the MOA system employs.
The other system that has been in use for a long time is Minute of Angle (MOA). Typically the MOA system employs either 1/4 or 1/8th MOA incremental change in the turrets and the reticle has either 1 or 2 MOA sub tension lines within the reticle.
This is all great but how does this affect me?
The easiest way to put this is the following:
1 Milradian equals 3.6 inches at 100 yards or 36 inches at 1000 yards. The smallest incremental change you can make is .36 of an inch at 100 yards or 3.6 inches at 1000 yards.
1 MOA equals 1 inch for all intents and purposes, it is actually 1.04" at 100 yards. In most scopes the unit of incremental change is 1/4 MOA, which becomes 1/4 inch at 100 yards or 2.5 inches at 1000 yards. Some scopes offer 1/8th MOA turrets so now your incremental change is 1/8th inch at 100 yards or 1.250 inches at 1000 yards.
Most of today's real precision shooters opt to use MOA simply due to the finer or smaller incremental changes in point of impact that they can adjust their scope to.