Arch-length deficiency
There is less room along the arch than the teeth need, so they overlap, rotate and sit out of line.
Crowding is arithmetic before it is anything else: the sum of the tooth widths exceeds the length of the arch that has to hold them. The deficit is measured in millimetres at the consult, and it decides everything that follows — under about 4 mm we can usually create the space by expanding the arch or reducing enamel contacts, and above it we are talking about extractions.