This article includes a description of methods to use to add a new outlet from an existing outlet. There are times that you may wish to install / add an additional outlet at a new location, now you need to figure out where there is an existing source of power that you can tap into.
All modern wiring should have a available equipment ground, and switch locations will have an available unswitched hot [for single switch systems, 3 way and 4 way switch systems may not have a unswitched hot available]
So, in summary, a switch location may or may not have all the necessary wires available that you can tap into. However, most all outlet receptacles will have all the necessary wire (except if the outlet is a switched outlet - both halves switched)
Therefore, in most instances an existing outlet is a good choice in getting a power source to run a new wire cable to a new outlet addition.
The silver color screw will have the grounded leg [aka neutral] attached to it (white or grey wire), the brass color screw will have the hot wire attached to it, the green screw of the outlet will have the equipment grounding wire attached to it (usually a bare wire but grounding wires can be bare or green)
To add a new outlet run a new cable between the existing outlet and the new outlet. Most cases you would run a black / white / bare wire cable, enter the existing outlet's electrical box, make sure after you strip away the outer wire cable jacket that you have a min. of 6 inches of wire inside the box, do the same at the electrical box at the new location.
At the new location, hot wire (usually black) gets connected to the brass color screw, the neutral (usually white) gets connected to the silver color screw, the bare wire gets grounded to the grounding screw of the electrical box (if a metallic box) and the grounding screw of the outlet (green screw).
At the existing outlet, if there was previously only 1 cable in the box (and therefore only one silver screw and one brass screw used) then you can either add the new cable by adding the white wire to the unused silver color screw, and black to the unused brass color screw. The bare wire to the 2nd grounding screw in the electrical box (if metallic and a second screw exists), or you can use the pigtail method.
If there is already more than one cable in the existing outlet, then you will have to use the pigtail method. However bear in mind there is a limit on how many wires are allowed in a given size of electrical box.