There are three versions and I will try to keep this as simple as possible.
Here are the three versions:
1) Cash Register Plus AKA QuickBooks POS Free
This allows you to turn your pc into a cash register. It tracks sales and can transfer the end of the day information into QB Professional Financial.
2) QuickBooks Point of Sale V10, Basic Edition
Same as Free except it tracks a value for your inventory. It allows you to receive inventory at cost, allows you to charge to customer accounts and this information can be transferred into QB Professional Financial.
3) QuickBooks Point of Sale V10, Professional Edition
Same as Basic except it allows you to track customer rewards, employee time & commission, and track inventory reorder points. It also allows you to create Layaways, Sales Orders, and Work Orders. It is also connected to constant contact.
Most people prefer QuickBooks Point of Sale Professional.