Complex sentence: Rat Terriers tend to be less aggressive than Jack Russells; while they have a definite terrier personality they also have an "off switch" and love lounging on the sofa in a lap as much as tearing about the yard.
Split into simpler sentences: Rat Terriers tend to be less aggressive than Jack Russells. They are a terrier by nature but they also have an "off switch" and love lounging on the sofa in a lap as much as tearing about the yard.