Transistors

Differences between NPN & PNP Transistors: "...an NPN transistor turns on when the current flows through  the base of the transistor... A PNP transistor turns on when there is no current at the base of the transistor [i.e. the base is connected to the ground]." Usually, the middle pin (#2) is the base but there are transistors like the TIP120 whose base is pin 1, check the datasheet.

You can connect transistors in parallel to handle currents that a single transistor can't.