Positional multi-axis machining (henceforth referred to as just "positional") is the most common way multi-axis CNC's are used today. It involves positioning/indexing one or more axes to a fixed location, after which the remaining axes are used while the original axes stay locked in place/plane
Programming positionally boils down to defining/picking a new plane, then proceeding with regular 2D and 3D toolpaths, perpendicular to the new plane
Positional is great because it can take a many-operation 3-axis CNC program/part and reduce it down to a 1- or 2-op part
For example, a 6-sided dice would take at least 6 operations using a 3-axis cnc, whereas the exact same toolpaths could be performed in just 2 operations using a 4-Axis Cnc Mill/3-Axis CNC Lathe, or just 1 operation using a 5-Axis CNC Mill/Mill-Turn
Additionally, positional is useful for machining along non-orthogonal planes (non-rectangular parts/features)
For instance, if you wanted to machine perpendicular to the 6 sides of a hexagonal part, this could be easily accomplished
Compound/complex-angles are also easily done with positional multi-axis
Simultaneous multi-axis machining (henceforth referred to as just "simultaneous") - as the name implies - involves synchronized, simultaneous motion of all axes on a multi-axis CNC machine
Simultaneous allows for machining of highly complex geometries not possibly or prohibitively complex with any other method