The boundaries are generally defined by the presence of Yucca (Joshua) trees.
The Mojave is an area of temperture extremes and has two distinct seasons--summer and winter. More often than not, passing storms bring winds and dramatic cloud formations rather than actual rain.
I've modified the image slightly and the newer version can be seen here.
Both inexpensive images and cards can be purchased on the site.
Click on the image to see a larger version against a black backdrop.
Enjoy and thank you for your comments.