Instructors are typically non-tenure track, have not completed a residency (after a Pharm.D. if in the practice department), may not have a Ph.D. or postdoctoral experience and are usually limited to teaching labs. Some places they may teach extra courses.
Depending on the school an instructor may be eligible to become an assistant professor after 5 years and some places instructors may never be eligible to become assistant professors unless they complete the residency, Ph.D., etc
An assistant professor is typically where most PharmDs with residency or PhDs start. After 5 years you are evaluated on your teaching, service and scholarly activity and move on to Associate...etc
I absolutely love my job and would not trade it for the world. There are some people on the forum who dislike (rather hate) academia but I thank the Lord every morning for my job. I absolutely have to report to no one (probably once a year), total autonomy, love learning and reading new stuff, super flexible, get money to do research, can take as much time off if needed for emergencies/children/etc. Having done retail and worked in industry, I can tell you that on a scale of 1-10 (10 being the highest) for how much I like academia, I can say it was around 2 for retail, 7 for industry and 10 for academia. I dont know if it is for everyone though and it might just be my personality, but I would work at my current job for exactly 1/2 the pay.