There are two types of rifle scopes when it comes to which focal plane the reticle is etched on and they are called first or second focal plane scopes.
What is the difference between the two types and why would one choose one over the other?
The technical difference is on which focal plane the reticle is etched on,
at the focal plane between the objective and the image erector lens system (the First Focal Plane (FFP)), or the focal plane between the image erector lens system and the eyepiece (the Second Focal Plane (SFP)).
While the practical difference is that a FFP reticle will increase in size when you increase the magnification on a variable magnification scope while a SFP reticle will stay the same size.
This mostly comes into play when you have a scope with a mil-dot reticlee for ranging targets/game. If you have an SFP scope, then it will only work at one magnification, while an FFP scope will stay constant.
For example, at 25 yards I need to aim 2 inches/8 MOA high. With an FFP scope, I can aim 8 MOA high regardless of the magnification power my scope is set to while if I had an SFP scope, I would be stuck at trying aim 2 inches high at that looks different at depending on the magnification power the scope is currently set to.
The disadvantages of the FFP scope is that because of the reticle getting larger with more magnification you lose some precision compared to an SFP scope and secondly that they usually cost more.