Mrchntmarine
very detailed diagram. Where did it come from?
Looks like one of the various illustrations out on the net (image search "cargo boom drawing" and you'll see a tone of them).
There are similar drawings in several books, The Bluejacket's Manual, and Knight's Mondern Seamanship, just off the top of my head.
For a bit more detail, searching out the Boatswain's Chief/1c Manual describes the actions at a bit more detail.
For a quick overview, cargo booms are typically worked as a athwartships pair.
A given boom has four things going on at its far end.
There's the Lift which raises and lowers the boom vertically.
Then, there are two Vangs which orient the boom horizontally.
Lastly, there's the Hoist which sways loads up and down from the boom end.
Booms are usually worked in pairs.
One boom is spotted over the general center of the hatch, the other over the side to the pier. The Hosts are then connected to each other at the running end.
To get something out of the hold, you drop the hatch Hoist while slacking the other hoist, the hook will drop down into the hold to where it's needed. You bend on the cargo, and heave the hatch Hoist until the load is clear. You then take in the slack on the dockside Hoist until the load veers from under the Hatch boom to the dock boom. Lowering away puts the cargo on the dock.
Here's a video that might help (silly landlubbers call vangs "guys" and hoists "lifts")