What happens where tectonic plates slide past each other?

Picture the boundary as a line between two neighboring plates, viewed from above. At a transform boundary, each plate moves horizontally along that line, in a different direction from its neighbor. The plates are not principally moving apart across the boundary or pressing toward one another across it. USGS describes transform boundaries as zones where plates slide horizontally past each other. [S1]

Here is a simplified overhead diagram. The arrows show motion; the drawing is not to scale:

                 OVERHEAD VIEW

       Plate A          │          Plate B
         ↑              │              ↓
         ↑              │              ↓
         ↑              │              ↓
                        │
              transform boundary

The line is the boundary, and the arrows run parallel to it. The sketch shows the basic geometry, not the detailed shape of any particular fault or plate boundary.

Compare the direction of movement relative to the boundary:

1. Transform: plates move sideways along the boundary, past one another.
2. Divergent: plates move away from each other; new crust forms in the general classification.
3. Convergent: plates move toward one another; crust is sometimes subducted. [S1]

A quick way to read a sketch is to look at whether the arrows run along the boundary (transform), point away from it (divergent), or point toward it (convergent). The difference is the boundary geometry and motion; the labels do not mean every boundary has precisely the same features.

Yes. These overhead sketches put the boundary in the center. Arrows are schematic, and each pair represents the plates’ general motion:

TRANSFORM — motion along the boundary
Plate A       │       Plate B
   ↑          │          ↓
   ↑          │          ↓
             boundary

DIVERGENT — motion away from the boundary
Plate A       │       Plate B
   ←          │          →
             boundary

CONVERGENT — motion toward the boundary
Plate A       │       Plate B
   →          │          ←
             boundary

For the transform case, USGS says these boundaries do not produce or destroy crust in the general boundary classification and are generally associated with shallow earthquakes. Some occur on land, including California’s San Andreas fault zone. [S1] These are broad descriptions: the source does not establish that every transform boundary has identical features or that a particular boundary will have a particular event.

Try: “At a transform boundary, neighboring plates move horizontally past each other along their shared boundary; unlike divergent and convergent boundaries, they are not moving apart or toward each other across it.”

To check your explanation, trace the boundary with your finger: if the motion arrows follow its direction, the sketch represents transform motion. Then distinguish the other two by checking whether their arrows point away from or toward the boundary. Keep the terms “generally” and “in the general classification” when mentioning earthquakes or crust, because those summaries should not be treated as guarantees about every location. [S1]