Crystals are solids that are made up of particles such as molecules, atoms or ions, which are arrange in a highly ordered microscopic structure, forming a crystal lattice which extends in all directions.
Examples of crystals include quartz, that is made up of silicon (iV) oxide molecules that are arranged in a lattice,table salt is another example that is made up of sodium and chloride ions that are arranged in a lattice
Crystals are very ordered, they may be made up of atoms of the same elements or atoms of different elements such as in the case of table salt.