Zerg's planet of origin. Remnants of the original Zerg (Primals) remain unscathed from Amon's corruption and those remnants repopulated the planet. Kerrigan later visited the planet and took control of the Primal Zerg by becoming a Primal herself.
Zerus is not for the faint-hearted, it is a volatile planet: frequent irradiation, asteroid bombardments, ever-changing surface (extreme weather conditions plus unforgiving landscapes). It is no surprise the things living there are bizarre as well, at times you don't even know which is flora and which is fauna. Also both tend to be very deadly - it's a dog eats dog world. Definitely not a place to go for a holiday unless if you want to evolve while you're on a break.