XenServer 6 introduced vApps - maintained with v 6.5
A vApp is logical group of one or more related Virtual Machines (VMs) which can be started up as a single entity in the event of a disaster.
The primary functionality of a XenServer vApp is to start the VMs contained within the vApp in a user predefined order, to allow VMs which depend upon one another to be automatically sequenced. This means that an administrator no longer has to manually sequence the startup of dependent VMs should a whole service require restarting (for instance in the case of a software update). The VMs within the vApp do not have to reside on one host and will be distributed within a pool using the normal rules.
This also means that the XenServer vApp has a more basic capability than e.g. VMwares vApp or MSs Service Templates which contain more advanced functions.