Acceptance testing is largely about the client accepting the final system (so that they will then pay the bill). As stated above, it would likely include a variety of different tests (volume, simulated, live, etc), however the aim of acceptance testing is to confirm that the original requirements have been met and hence the system is complete and functional. Often tests are constructed to specifically confirm that each requirement has been fulfilled. For instance, particular response times may have been a requirement - this is tested and if within sepcifications the requirement is accepted by the client as having been achieved. Each requirement is specifically tested and if all have been met then the client accepts the system as complete. For larger systems an outside testing organisation may perform the acceptance tests on behalf of the client and for smaller projects the client may perform the acceptance tests alongside the developers.
HTH
Sam