what's entailed in "due diligence"

[Marci Stein]

when i read "make sure you do your due dilence" what exactly does that mean? 

I know to have  the home inspected, and make sure title is clear

 (part of lawyers job),but anything I'm missing?

for examlple, what steps do you take in the due diligence process when buying from  

auction.com?