A sprue is a hole in the mold where the molten metal enters. On the outside it is funnel shaped and on the inside it connects to the gate or gates, sometimes called a gating system. There can be a deeper part at this junction to trap any loose sand before the metal enters the gating system.
These are the channels or runners that connect the sprue to the pattern or mold cavity. They can by dug by hand after the mold is opened to remove the pattern or they can be part of the pattern, as with a match plate. If dug by hand, care must be used to smooth them and remove any loose sand that may wash into the mold.
A riser may look like a sprue, but it has a different function. Instead of metal being poured in, it rises from the mold cavity as the sprue is filled. It tells you when to stop pouring as well as provides molten metal to thick parts of the mold to prevent shrink cavities as it cools.
In the photo above, the flywheel is in the "as cast" condition. This is what it looks like when removed from the sand. The sprue is on the right and is connected to the flywheel by the gate or runner. The riser is sticking up from the center. Casting is ZA-27 and belongs to Ray Ethridge of Houston, TX.