Lice are spread primarily by head-to-head contact with an individual already infested with head lice.
Lice can't jump, swim or fly, so the head-to-head contact has to be close enough for the louse to grab onto the new host's hair while holding the old host's hair with one of its other six claws (which you can clearly see in these lice pictures).
Less frequently, lice are also transmitted between people by head-to-hand contact and by items such as hats, hair ties, scarves, pillows etc. However, scientists disagree on how often this type of "fomite" transmission (i.e. transmission from an object contaminated with lice) really occurs.