The 555 Timer uses 3 main functions: Astable, Monostable, and Bistable. The Astable outputs a high pulse on a steady repetitive pace. The Monostable when triggered holds the output high for a predetermined time period. The Bistable can be held high and low by pressing the pushbuttons. If you want to learn more, visit RealCircuits.com.
Step 1: Astable 555
This build can be used in a variety of circuits: Timers, clocks, drive servo motors, and IR Sensors. The Astable is able to do this because it can change the cycle from milliseconds to minutes using two resistors and a capacitor.
Step 2: Monostable 555
Using the monostable, you can output a high pulse for a preset time period. Using a capacitor and resistor, the time period can vary from milliseconds to minutes.
Step 3: Bistable 555
Another way to think about this circuit, is simply as a switch. If you press the bottom pushbutton the LED will go high and pressing the top pushbutton it will stay low.