A "transform fault" is a fault line where two tectonic plates slide against each-other. Not all faults are transform faults. Some plates separate and allow magma to seep up and like a scab, creating new ground (these are called divergent faults.) Other faults, such as convergent faults, occur when two plates smush into each-other and one is forced below the other where it heats up and is eventually destroyed. Extreme examples of these types of collisions result in the birth of mountain ranges.
Do you see the difference?
Transform faults are the "crack line" where the two plates slide against each-other going different directions. This, of course, does not happen smoothly; force is built up over a long time and then the plates "slip" in hiccup like motions which cause earthquakes.
Most transform faults are found on the bottom of the ocean, but a famous example of one above sea level is the San Adreas Fault in California. Here is a picture of how the faults work. The transform fault is in red.
A transform fault is the fracture zone around a transform plate boundary. A transform plate boundary is the boundary between two tectonic plates where the two plates are sliding past one another (instead, for example, a place where one plate is sliding under the other).
Although most transform faults are found under the sea, the most famous example of a transform fault is the San Andreas Fault in California. As is the case with this fault, transform faults are quite often the centers of a great deal of earthquake activity.