Expense vs Bill

Expense vs Bill

What's the difference between an expense and a bill? When should I use them?

Eg. I charge a client for a service.  I charge them 1 lakh for the job. However, I also have to pay 3 guys @ 25000 for getting it done. So I've spent Rs. 75000 for getting this job done. Is Rs. 25000 an expense or a bill? 

P.S. 
I do ask the 3 guys to send me an invoice for the services they provided to get this done.