Every wizard can cast a handful of cantrips each day. You don't have to memorize or choose them beforehand, you just cast them on the fly.
Wizards can cast a number of cantrips equal to their Intelligence modifier each battle. Each cantrip takes a standard action to cast as a ranged spell. Outside of battle, a wizard can cast about three to six cantrips every five minutes. The Cantrip Mastery talent speeds up cantrip casting.
At the adventurer tier (levels 1–4), cantrips with a standard duration last 10–60 minutes, plus 10 minutes per wizard level. The GM rolls and the wizard becomes aware that their cantrip is about to end a couple minutes before it's done.
At the champion tier, levels 5–7, most cantrips last 1–6 hours.
At the epic tier, levels 8–10, cantrips last between 2–12 hours.
For a list of available cantrips, see Cantrips.