Catnip is a perennial plant. Like other members of the mint family it has square stems and opposite leaves. Catnip is highly favored by cats, they eat it and get kind of goofy and playful.
Leaves (below) are typical, but the white coloration or sector of the leaf in the middle is caused by a mutation of the chlorophyll genes in that part of the leaf. This is known as a chlorophyll leaf chimera mutant.
White flowers on a catnip seedhead: