I'll try to explain without getting too long winded.
You have Enlisted and Officer.
Enlisted start at E-1 in Boot Camp (generally fresh out of HS). NCO's (Non-Commissioned Officers) are Enlisted equal to or above the rank of E-4. Sergeant is E-5. A SNCO (Staff NCO) is equal to or above E-6.
Officers start at O-1. They are the ones that went to college first and got a Bachelors in something. Lieutenant, Captain, and Major are Officer ranks (and in the correct order).
There are Officers who started out as Enlisted. Field Promotions however, I don't think occur very often anymore (if at all).
In my sector of the corporate world, enlisted are up to and including supervisors, while Officers are Managers on up to CEO.