There are three types of slalom:
1. Giant slalom - Usually against the clock. In the giant slalom the cones are placed farther apart and offset from the fall line.

2. Tight slalom - Head to head or against the clock. Cones are close together and along the fall line.

3. Banked slalom - Usually against the clock. Done at skateparks with long ditch-like runs, or in similar locations. Skateboarders ride up and down the banks as well as through the cones.

Fall line - an imaginary line down the center of the slalom run. Usually cones are placed along this line. It is best to stay as close as possible to this line in order to achieve faster runs.

Offset - Sometimes cones are placed on one side of the fall line. This is called an offset cone, or sometimes called an offset gate.

Pump - moving the hips, waist, arms and body in a manner that causes the board to turn back and forth. Pumping helps slalom skateboarders gain speed, even on flat ground.

UPDATE:
This revised definition of Fall Line was submitted by Peter Camann:

Fall line - an imaginary line down a hill where gravity's influence will determine the roll or movement of an object or individual. Example: if one stands at the top of a race hill and rolls an object down the course, the object will take the path of the fall line. Usually cones are placed in an offset pattern along this line creating speed with the grade along the fall line. However, often streets or terrain have grades with a double fall line, i.e. downhill and sidehill. Whenever this type of fall line is present, speed can be both increased and checked (for purposes of control or challenge) when the course pattern is set across or against the grade of the sidehill's fall line. In skiing, the fall line is defined as the natural line of descent.