Sinking and sourcing current
In a very simple form, relative to VCC, think of the transistor as either coming before or after the device.

If the transistor is connected between VCC and the device, it is sourcing current.

If the transistor is connected between the device and ground, it is sinking current.



(Image from CircuitsToday.com)

Some articles that describe things in more detail:


National Instruments: What Is the Difference Between the Terms Sinking and Sourcing?
dataq.com: What's all this Sink and Source Current Stuff?