This circuit was a very simple light, with power provided by the Arduino board through pin 13, as seen in both the schematic and the code.  The code too was very simple, being an infinite loop that blinked the light on for a second, and then off for a second, and then repeated. 

This circuit was almost identical to Circuit 1, with one exception. A potentiometer was added, which sent analog data back to the Arduino. This analog data was read by the Arduino, and allowed it to adjust the rate at which the light blinked. The ability to adjust an output with a potentiometer is very useful for future projects, as it allows an analog input, rather than a binary on or off, as seen with switches. 

This circuit allowed the manipulation of the specific color of a light, but modifying its RGB values. As far as hardware goes it was very similar to the first circuit, with the exception that the RGB LED requires three input pins, one for each color. By changing which diodes turned on it was possible to create multiple colors with a single LED, however, due to the binary nature of the individual diodes the amount of colors that could be created was limited.

This circuit was basically the first circuit repeated 8 times (4 shown in schematic). While the hardware was not different in any noticeable way (other than there being more of it), using multiple LEDs allows for several uses, from displays like those on kitchen clocks to the blinking lights so often seen at move theaters and carnivals.

This circuit saw the introduction of buttons for the first time. Buttons are of very high utility in circuit design, as the relate directly to binary, they are either on or off, either 0 or 1. This makes them useful for all situations where more complex analog inputs (such as provided by the potentiometer as seen above) would be unnecessarily complex, such as, but by no means limited to, opening doors, turning devices on or off, single value inputs (like keys on a keyboard), etc. This circuit also saw the introduction of logic gates, with these buttons being connected in an OR gate. Logic gates are hugely important, and are where computing really begins. Thus, learning to use them is crucial for any aspiring engineer.

This circuit saw the level of light emitted by an LED directly tied to the amount of light detected by a light sensor. While in this form this circuit presents only limited utility, if the code is reversed then many different functions can be achieved. For example, car lights could be made to automatically drop when another car's headlights are detected, as to avoid blinding other drivers with your high-beams. Or outdoor lights could be set to come on only when it gets dark, saving energy and bulb wear. Overall, though at first this circuit seems to offer only limited real world applications, if a little code is switched it becomes a very common circuit in our daily lives.