A snapshot is a photo of a state of a virtual machine. As a virtual machine often requires a lot of work before a desired state of the machine is reached, it is a good idea to take a picture of that exact state. If something goes wrong at a later stage, having a snapshot makes it possible to easily revert to the previous state of the virtual machine. So the base concept of working with snapshots is to make it easier to revert to a previous state.
A clone is a copy of a virtual machine. Using clones is convenient if several virtual machines are needed, with more or less the same configuration on each virtual machine. By cloning a virtual machine, you'll make a copy of the actual state of a machine. After making the clone, you'll just have to modify the properties of the virtual machine that need to be unique on that machine.
In some ways, clones and snapshots are closely related. This is because you can create a clone of a snapshot of a virtual machine...