What's the difference between a ticket and a boarding pass?
After paying for your booking, you'll receive a ticket. If you book online, you'll receive an electronic ticket (e-ticket); if you book at a travel agency or ticketing office, you'll receive a paper ticket.
The ticket shows the full journey you've purchased. Each ticket has a number; Brussels Airlines tickets always start with 082-21 followed by 8 digits. One ticket is issued per passenger so if there are 4 passengers flying, there will be 4 tickets.
You need a boarding pass to board the aircraft. You'll receive it after checking in for your flight. If you check in at the airport, you'll receive a paper boarding pass; if you check in online, you'll receive an electronic boarding pass.
For Brussels Airlines flights, online check-in opens 24 hours before departure.