Diodes have two terminals, but current can only pass through them in one direction. A diode is indicated by an arrow parallel to the wire line, and a line perpendicular to it. Current flows in the direction the arrow points.
A light emitting diode (LED) is a diode that also produces light. Its circuit symbol is the same as a normal diode, but with two smaller arrows pointing away from the main arrow to indicate emitted light.
On a Zener diode, both terminals have the same voltage. The symbol is the same as a normal diode, except the line has a small “serif” on its top.
A photodiode can react to light. Its symbol is the same as the LED, except the small arrows point at the main arrow, to indicate light coming in.