Baggage checked from Beijing to Toronto (via Chicago) on United. Will I need to collect luggage in Chicago and re-check for final leg from Chicago to Toronto?
Yes you will have to collect and recheck baggage. You will go through US Customs and Border Protection too.
Step 1 : Arrive at O'Hare International Terminal 5
Step 2 : Proceed to Customs and Border Protection (CBP Processing Area - Immigration Inspection
Step 3 : Claim Baggage
Step 4 : Proceed to CBP Processing Area - Customs and Agriculture Inspections
Step 5 : Connecting Flights/Transfers
Here is a really neat visual guide
Yes. When you land in Chicago you will go through US passport control. After that point, you will need to collect your bags prior to rechecking them on the other side.