AppVirt is short for "Application Virtualization". And ever since Thinstall and SoftGrid emerged on the scene, the attention has spurred a party with more imitators showing up by the minute. Some might say the party's already begun, but that wasn't really a party, yet. That was people showing up and setting up things and stacking up the cups for the pending brew-fest to come.
Here's a really well-done review of Symantec/Altiris SVS technology. I love how each vendor scrambles to come up with a clever name for the same concept (to avoid trademark litigation no doubt). SVS calls their client agent memory stack "the Layer". SoftGrid has "system guard", and VMware casually refers to ThinApp's logical isolation boundary as "the Sandbox" (but you have to dig to find that stated anywhere), but also as the "virtual OS" in some instances. I'm not sure what Citrix calls their XenApp bubble, but I'm sure they at least have a pet name to use around the water cooler.
There are striking similarities among the current AppVirt. The most common being how they redirect the memory stack, folder and registry requests and shortcuts. Where they differ mostly seems to be in how they support preparing and delivering a package. SoftGrid, for example, provides a client-server pairing whereby the server offers "streaming" to improve delivery performance and scalability. ThinApp doesn't provide any dedicated server, it just focuses on building a package that is incredibly portable and self-sustaining. SVS appears to offer something in between, still making use of a "server" component in the overall scheme.
A really good comparative review of many/most of the current products can be found at http://4sysops.com/archives/application-virtualization-products-the-complete-list/