The choke is a constricted portion at the end of a shotgun barrel that helps make the shot patterns smaller. The tighter the choke, the smaller the pattern. If you were hunting you would want a tight choke for far away game (say geese) and an open choke for close game (say rabbits or quail). While there are variations, the normal range of chokes is:
Cylinder: No choke, say for self-defense and other very close situations.
Skeet: Very slight choke, for the game of skeet or close up game birds.
Improved Cylinder: Tighter than above, but still mild. Good for close up game, informal clay targets, etc.
Modified:: Tighter than above. A good, average all-around choke
Full: A tight choke, good for far away shooting at game or for the game of trap. Turkey and coyote hunters sometimes use a 'super' full choke for even tighter patterns.
In the 'old days,' shotguns barrels were bored with the choke made into in the barrel. Most newer guns have interchangeable chokes that screw into a threaded portion just inside the end of the barrel. Double-barrel shotguns have traditional had a different choke in each barrel, the theory being the target is closer to you for the first shot and further on the second shot. Now, of course, with interchangable chokes, a double gun owner can do anything he wants.
The ventilated rib is a raised rib (on posts) that runs the full length of the barrel (on top). It makes for a nice, flat, and narrow sighting plane. Most modern shotguns (other than those made for self-defense) have ventilated ribs. A few decades ago, it was considered an option that increased the price of the gun.
