Diodes are components which are polarized. They only allow electrical current to pass through them in one direction. This is useful in that it can be placed in a circuit to prevent electricity from flowing in the wrong direction.
Another thing to keep in mind is that it requires energy to pass through a diode and this results in a drop of voltage. This is typically a loss of about 0.7V. This is important to keep in mind for later when we talk about a special form of diodes called LEDs.
The ring found on one end of the diode indicates the side of the diode which connects to ground. This is the cathode. It then follows that the other side connects to power. This side is the anode.
The part number of the diode is typically written on it, and you can find out its various electrical properties by looking up its datasheet.
They are represented in schematic as a line with a triangle pointing at it. The line is that side which connected to ground and the bottom of the triangle connects to power.