Simple resource specification, resource selection, and effective binding are critical capabilities for Grid middleware. We describe the Virtual Grid, an abstraction for dynamic grid applications to deal with complex resource environments. Elements of the Virtual Grid include a novel resource description language (vgDL) and a resource selection and binding component (vgFAB), which accepts a vgDL specification and returns a Virtual Grid, that is, a set of selected and bound resources. The goals of vgFAB are efficiency, scalability, robustness to high resource contention, and the ability to produce results with quantifiable high quality. We present the design of vgDL, showing how it captures application-level resource abstractions using resource aggregates and connectivity amongst them.