tpyically for public work jobs we have to submit with our bid a 5% bid bond.  This essentially is your gaurantee to the awarding authority that you will sign a contract within 5 days of being presented with one at your bid price or they can call in your bond and get 5% of your bid.  The bonding company would pay out and of course comes after you for the money.  
once you have a contract, the GC or awarding authority may require a performance and payment bond -  this bond gaurantee's that you will actually complete the work and you will pay all your bills related to that job.  If you walk away from the job 1/2 way through the bonding company may be responsible to finish the project or may pay out in cash the amount needed to complete the job.  Again - they come after you for the money they pay out.  
I like having bonds because it protects me that the GC will pay me for the job as well.  
The bid bonds are cheap money - kind of free - we pay a yearly fee to be able to get an unlimited amount of bid bonds.  the P&P bonds at like $30 per 1k for us.