Calottes (also know as clam shells and bead tips) cover the knot at the end of a string of beads. They hide the thread end knot and provide a link to attach a clasp to.
Crimps are small metal beads that are crimped (squeezed) on to a thread and hold beads in place to make illusion type jewellery or can be used at a necklace ends to hold clasps in place.