In the UK a 'junction box' has threaded brass slots, you insert the wires into the slot and tighten the screw down on to the wires. Far more secure than simply twisting two wires together and covering them with a plastic nut.
Someone actually posted a picture of one in the previous thread, although it may have been closed.
These though are fairly rarely used, most UK wiring is directly from the fuse box to where it needs to go, mains sockets are done on a ring main, usually one for each floor - where the wire goes from the fusebox to each socket in turn, then back to the fuse box. This gives double the current handling capacity that a simple spur connection can - you use 2.5mm twin and earth cable, rated at 27A (in free air), giving 54A worth of cable on the ring.
Lighting circuits are done using thinner cable, usually suitable connections for switches are part of the light fittings, but if not you would use junction boxes to provide the required extra connections for the switch.
Probably the main use of junction boxes is tapping in to existing cables to add extra sockets or lights.
Here's a picture of a junction box that's open: