It’s a virtual sample library and an instrument plug-in.
Each Plugsound volume is a virtual instrument based on a professional sample library, available as a VST2 plug-in (Mac-PC), as well as a MAS and RTAS plug-in on the Mac. The concept is that when you buy Plugsound you buy a sound library. But instead of getting a CDROM for your inadequate sampler, still as slow and annoying to use as 10 years ago, you can use the sound library directly in your sequencer.
If you’re on PC you’ll need Cubase, Logic or another VST2 compatible sequencer. Owners of Sonar should use DirectiXer™ to use Plugsound.
On Macintosh, any VST2 program like Cubase or Logic, but also Pro Tools and Digital Performer will allow you to enjoy Plugsound.