Diodes
Diodes are are the one-way arrows of electronics. Drawn sort of like an arrow, they allow current to flow only in the direction they're ponting.
When you look at a diode, there will be a stripe around one end. This stripe is like the bar that the arrow points to. Current flows towards the stripe. So, it's important to pay attention to which way you put the diode into your breadboard.
Normal diodes are used for rectifying signals and power supplies, and for clamping signals to a certain range. With an op-amp, a diode can be made to calculate logarithms. Finally, zener diodes are special diodes that can be used to establish convenient voltage levels.