A lender is a financial institution that makes loans directly to
you. A broker does not lend money. A broker finds a lender. A
broker may work with many lenders.
Whether you use a broker or a lender, you should always shop around for the best loan terms and the lowest interest rates and fees.

How can I tell if I am working with a mortgage broker or a mortgage lender?

Some financial institutions operate as both lenders and brokers, so you should ask whether a broker is involved in your loan transaction. 
Most brokers are paid a fee for their services on a specific loan. The fee may be in the form of a commission or other payment from you or the lender.

Just as with lenders, you
can and should shop around for brokers.