The original boards were well over 3.5m long, weighed a good 15-20kg and had an awesome amount of volume. Don't know exactly what volume but a 90kg+ man could stand on them without appreciable displacement.
Boards had a centre board as well as a fin. The centre board was similar to a dagger board on a dinghy. Essentially, it countered the lateral force of the sail and allowed the forward component to dominate -> forward motion. The fin was more to assist steering like on a surfboard / paddle ski.
Early masts were fibre glass and usually one piece. They had a little flexibility and were attached to a mast extension of sorts, which attached to the board via some time of universal joint which allowed the mast to move and rotate in all directions.
The boom was a very long double sided wishbone which attached to the mast via a complex tieing on procedure with thick rope. The trick used to be to tie the boom on whiel it made an acute angle with the mast in such a way that when it was pulled to a 90 degree angle, the knot would tighten around the mast and keep it from sliding up or down. This wasn't a particularly stable arrangement.
Sails were typically triangular and made of sail cloth with a vinyl window for looking where you were going. They had a mast sleeve through which the mast would slide and downhaul and outhaul grommits at the foot and clew respectively. The downhaul grommit was tied firmly to the mast base and the outhaul grommit to the end of the boom, thus securing the boom in place and the mast on the mast base.
That's about it. Oh, almost forgot the crucial uphaul rope. Recall that the Universal Joint allows the mast / sail / boom (ie rig) to move in any direction so, of course, when starting out with the whole contraption in the water, the rig would float next to the board. In order to lift the rig up into the wind, an uphaul rope was attached to the boom near the mast and the base of the mast.
Right, so all the sailor had to do was push this lot out onto the water, clamber onto the board, haul the rig up out of the water, grab the boom and voila - sailing.
Steering was achieved by tilting the rig either forward or aft. In other words, while keeping the mast over the centre line of the board (imagine a line drawn from the nose to the tail down the middle of the board) the mast (and rig) were tilted either towards the nose or tail of the board. Tilting the rig toward the nose caused the nose to move downwind (ie bearing off in sailing terms) and tiliting it towards the tail caused the nose to move upwind (I forget the sailing term for that ;-)
Actually, it was probably the foot pressure of the sailor which caused that to happen but the easiest way to teach it is to perform the sail tilt. It results in the same thing.
So, what has changed ?