As mentioned , primary partitions are drives for loading the operating system,logical drives are non operating system partitions for loading your programmes.
Hidden and unhidden refers to a system whereby you would normally load more than one operating system for dual boot etc.
In this situation there would be 2 or more primary partitions one for each system.
Depending on which one you boot to, the other(s) would be hidden to save confusing Windows .
Bootmagic (Partition Magic) therefore hides and unhides partitions on multiple operating systems automatically depending which one you load.