Transistors are powerful little electronic switches, and they really like these NPN transistors whenever they need to control medium-power electronics such as small motors, solenoids, or IR LEDs. Adafruit find them so handy, they come in a pack of 10!
Each transistor is a general purpose amplifier, model PN2222 (same pinout as the 2N3904) and has a standard EBC pinout. They can switch up to 40V at peak currents of 1A (not continuously, just peak!), with a DC gain of about 100. They're basically your garden-variety NPN, and do the job with class! For more details, check the datasheet.