In this exercise, a transistor (type NPN) is used to amplify power to an LED. On the transistor itself, its “collector” is connected to PWR, “base” to a momentary pushbutton (type SPST), and its “emitter” to an LED.
In a nutshell, the “collector” provides a steady source of PWR. The “emitter” can only send power out when the “base” is given power. When the pushbutton is pressed, it allows power to go to the “base”, which causes power to amplify and be sent out via the “emitter”. The transistor acts as an amplifying switch.
If it was a a type PNP transistor, the “collector” would be connected to GND, the “emitter” to PWR, and the “base” to GND. In order to have power go from the emitter to the collector, the base must have negative voltage.