An ensemble library groups sections of the orchestra into one single playable patch. For example, the Low and High String patches in Abbey Road One. Each patch contains what the name implies, the lower range of a strings section's instruments in Low Strings, and the higher range in High Strings.
Why are they useful?
Aside from saving time, ensemble libraries are a great starting point for those that aren't quite ready to program an entire orchestra. The sections are also pre-mixed to industry standards as well.