What is getting splinched in Harry Potter when you apparate?

Apparating is transporting from one place to another. The wizard or witch must have a wand to apparate and he or she must concentrate on the place he or she wishes to go, unless "splinching" will occur (splinching is when part of the body is left behind because the wizard wasn't focused enough to get his entire self to his destiniation). Side-along Apparation is when one or more wizards accompany or grab onto the wizard apparating and are transported along with him.

Harry describes the sensation as being squeezed through a tiny tube, being unable to breathe during that time, before popping in at the desired destination.

To apparte, a witch or wizard must have the coordinates, (Latitude and longitude) or must have been to their destination before, so they have a mental image.