What are Laravel queues connections and onQueue method?

I just finished to read laravel documentation about queues and I dont understand somethings.

1) What are these queues connections ?
What they do?
What is the different between 'beanstalkd' , 'sqs', 'iron' and 'redis' ?

2) What onQueue method does?
I red that you can with named queues tou can categorize the workers, but what the workers are? jon classes?
For what it's good?

via Chebli Mohamed