So first a little of the history of both products Microsoft purchased Softricity back in 2006 which sold a product called Softgrid and it has been renamed and enhanced by Microsoft and is now called App-V. What App-V provides is application virtualization and application streaming. So what this means is that you can stream and run Access 2003 and Access 2007 at the same time on a Windows client or server. App-V will also attempt to do intelligent re-sequencing of the application that it streams to a client so that the application can be started before the entire application is stream (sent) to the client device. There is also an upper limit of 4GB for the maximum size of the application that can be streamed in App-V. Now for Citrix XenApp which confuses me sometimes since they re-named the products it was originally called Citrix MetaFrame Server and Citrix Presentation Server. And it also provides presentation virtualization (so Windows Terminal Services) and application streaming where you can publish applications on the Citrix XenApp server that endpoint client devices can run. So if you are looking just Windows application virtualization then App-V is a pretty good choice. But if you are looking at both Windows and Unix/Linux application virtualization then you will need to go with Citrix XenApp.