Short answer: Broker shops multiple banks, usually free. Direct bank is simpler but limited to their products.
A mortgage broker shops multiple banks for you. They know which banks lend to people like you. They are usually free (paid by the bank). Going direct to your bank is simpler but you only get their products. If your situation is unusual, a broker saves time and money.
Broker Options: 4 banks, 6 products
Direct Bank: 1 bank, 3 products
Broker Fee: $0 (lender pays)
Time Saved: 2-3 weeks of research