That is the basic, structural difference, the important difference is in the effectiveness of each type.  Room air purifiers tend to be more efficient, easier to maintain, and more convenient.

More Efficient

Room air purifiers tend to be more efficient for a number of reasons.  For starters, most of these units feature multiple air cleaning technologies.  By this we mean they might use HEPA filters, electrostatic precipitators, and an activated carbon filter all in one unit.

Further, good room air purifiers create sufficient air flow to exchange the air in a room 4-6 times per hour.  Most central air purification systems will only turn the air over twice.

Easier to Maintain

Most people will agree that changing a filter in a stand alone room air purifier on the ground is easier than getting up into the ventilation system of a home.  It also tends to be easier to inspect the filters and devices on a portable air purifier versus the whole home unit.

More convenient

Another benefit of room air purifiers is convenience.  Portable air cleaners can be moved from room to room as conditions warrant.  They can also be placed into different parts of room to discover which location will be the most effective (as far as creating strong airflow).

Overall, most people prefer to go with a room air purifier when purchasing an air cleaner.  The only thing that keeps some people from using this type of unit is the cost.  If you want to clean the air of an entire home, a whole home air purifier may be less expensive than buying a separate air purifier for each room.  A portable air cleaner will only clean the air in one room or open area at a time.