The second pic is probably the most helpful. The cope is the top half of the mold and the drag is the bottom. You can see how the two risers are used to both vent air and allow for a "tank" of metal to feed back into the part as it cools. It also shows how you can use two halves of a part model, one in the cope and one in the drag, so that the parting line is in a place that you can get your part out of the sand. Well, you are going to have to bust the cooked sand off the cast part, but you wouldn't be able to get the model piece out in order to make the cavity.
You can cast more than one part at a time by using several models and then connecting them with runners.
You could also pour directly into the part instead of using a runner between the sprue and cavity, but you could get turbulant metal flow and could cause problems.
That second pic should say sprue instead of runner, runners are horizontal flow paths, the sprue is the fill hole.