Retail POS systems typically fall within 2 categories: live on-line systems and batch/on-line systems. On-line systems require your internet or network connection from the store computer to the back office system to be active all the time otherwise processing is not possible.
Batch/on-line systems allow a store to continue to process sales regardless of whether there is an active connection between the store and back office accounting system.
POS systems vary greatly in their functionality in other areas such as the following:
Other areas to consider are the number of stores in your organisation and whether there are classes of stores and requirements to handle automatic order replenishment.