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Diode Zener Diode Light-Emitting Diode
Electrons flow against the direction of the arrow. The side with the stripe is called the cathode; the side without the anode. At voltages below the breakdown voltage, electrons can flow into the cathode and out the anode, but not the other way round.
Most diodes are cylinders, with one wire coming out of each end. The side of the diode with a stripe corresponds to the side of the symbol with a stripe. LEDs do not have stripes, but indicate the cathode using a shorter wire. The larger side of the symbol corresponds to the longer wire of the component.
The diode symbol may optionally be enclosed in a circle. This is merely a matter of preference. A diode with a circle is the same as a diode without.