What is the difference between the top mount and under mount?

To make it easier to understand, let’s show you the pictures.

Top Mount

As you can see, the granite sink sits on top of the stone top.

Let's see another top mount example.

Cooker hob is always top mount.

Under Mount

The sink is under the stone top. Another words, the stone top sits on top of the sink.

To prevent water splash leak into the cabinet, normally we will do a overhang 5mm edge as shown in picture below:-

Conclusion

Top mount is easier to change in the future. But need to reapply the silicone when it's out of function especially it's dried and cracked due to fair wear and tear.

Undermount can't be changed at all. However, the silicone dried and cracked the water not easy to leak into the cabinet. If the situation is serious, you still fix by reapplying the silicone.