When speaking about current Sink or Source, this isn't just a specific voltage level with resporc to another. There is a big thing you leed to know about power supplies and regulators, and IC output pins.
If you take one of the simple power source, a battery. If the battery is 9V, and you put a load across the two poles, then the current flow through the load. The current will go in one direction. One pole could be refferenced as GND, the other as Vcc. However, if you take two batteries, one is the above 9V battery, the other is a 12V battery. You connect the negative poles togetter, let it be the GND refference. Then, if you connect a load between the two positive poles (one pin on 9V battery + pole, the other pin on the 12V battery + pole), then, currect will flow, but will go in the *other* direction, in the 9V battery, than in the first case.
When the 9V battery provide current, which goes out of 9V (using conventional current), the battery is said to *source* current. When the battery is absorbing current, it is said to *sink* current. Source and Sink really only does tell in which direction the conventional current flow.
Now, batteries are ideal power supply. This kind of power supply isn't usually found in electrical circuits. Many power regulators, converters, ... can only flow current in one direction. So, for example, a 7805 5V regulator can only *provide* current. It can only *source* current. If you replace the 9V battery in the above example, with this kind of regulator, no current will flow. Since no current flow through the load, by ohm's law, the voltage at the regulator output will be 12V (in the above example). Note that the regulator doesn't provide 12V, it come from the other voltage source. Such a case can cause problems in circuits, like blowing the regulator or other components.
So, you'll ofted see device specs about current *Sink* and *Source*. If the device tell that it can source (or *drive*) a specific current amount, but nothing is said about sink current, chances are that this device can only source current. It also often occur that a device pin can sink and source different current ammount. For example, open-drain or open-collector outputs, by design, can only sink current. A simple device that can sink and source current, for example, is an operational amplifier.
So, that's pretty much all about *sink* and *source* current specs.