They are holometabolous, i.e. they undergo complete metamorphosis with a distinct larval, pupal and adult stage. With the principal feeding as a larva, and the sexually mature stage as an adult. So every adult beetle you see was once an egg, a larva, a pupa, and finally the adult beetle.
Around 40% of all known insect species are beetles, this equals about 400,000 species and some estimates suggest there could be as many as 3 - 8 million beetle species on Earth.