Testing when development and testing are essentially completed and final bugs and problems need to be found before final release. Typically done by end-users or others, not by programmers or testers.
Beta Testing is release of a Software Product to a controlled number of users (Mostly Customers) and having a collective feedback from them regarding the product. This helps in realising the quality of the product and analying the defect & improvement (If any) required before the product is produced in mass and released on a full scale. Beta Testing is very usefull for COTS products.