Hi mate,
The XR is a trim level, like our SRi, SXi, Life, Club, etc in Europe. The Astra D, E, F, G, H and J are models of Astra. As the Astra is pretty new in the US, you've only ever seen the H on your streets, but we've had the Astra for decades over in Europe. These are the different models, and I've picked the top spec models for each picture (GTE, GSi and VXR):
Astra D / Mk1 (1979-1984):

Astra E / Mk2 (1984-1991):

Astra F / Mk3 (1991-1998):

Astra G / Mk4 (1998-2005):

Astra H / Mk5 (2004-present, 5 door production stopped 2009):

Astra J / Mk6 (2009-present):

The Astra J is so recent that it's only been on sale for about 3 months now.