How does public tax auction work?

[Tou V.]

Just wondering how does public tax auction work?  If I win at one of these auction do I get title (become legal owner) of properties?  Does the previous mortgages get cleared out, considering the lender had years to redeem the property and avoid the tax auction?  I've seen some houses going to auction for $20k and the winner bidding only $40k for a house worth at least $120k.  Anyone with experience with this?  Thanks.