The Enderpearl isn't only useful in singleplayer, you can also find them in the inventory of the robbers. And because of cops not being able to use sponges, the Enderpearls are an important item for them to arrest players with Crime. With the Crime-Texturepack the enderpearl has a different texture.
The usage is the same as in the singleplayer: Just rightclick with the pearl in your hand to throw the pearl in the direction in which you are looking at. If the enderpearl hits the ground the player will be teleported to this place where the pearl did hit the ground. If it hits the water the pearl will slowly sink down and teleport you to the ground of the lake or ocean.
You can buy them at the Pearlshop for 250 dollar. The Item-ID, which you need to buy this item is 368. Otherwise you can use the item-Name ender_pearl. But you can also get them for free: Kill the endermen which you can find in the mine.
Price: 250$ / ID: 368