In this project, you use both a 2N3904 NPN transistor and a 2N3906 PNP transistor, so you really need to keep track of which is which. The good thing about these types of transistors (especially compared to those troublesome photoresistors) is that their model numbers are stamped on their cases, so you can always tell at a glance which one is which.
Like the 2N3904 transistor, the 2N3906 transistor has three leads, also called terminals or pins. These pins connect to the base, collector, and emitter inside the transistor case, as shown here and indicated in the pin diagram on the product packaging or datasheet (check the Internet). The pin diagram for the 2N3906 transistor is the same as that of the 2N3904 transistor.
Depending on where you get your transistors, the pin assignments may or may not be the same as the pin assignments shown here. Check the packaging or documentation that comes with your transistor to determine which pin is which.
When you plug your transistors into the solderless breadboard as you build the circuit in this project, you need to make sure that you use the correct transistor (that is, 2N3904 or 2N3906) as instructed and that you orient the transistor leads correctly.