United Kingdom

Normally when you sign an employment contract it will say something like you work 37.5 hours per week between 9am and 5pm with 30 mins lunch break, or whatever.

A zero hour contract doesn't state how many hours you will work, so there's no guarantee you get any hours. One week you might work 40 hours, the next week you get nothing, and the next you only get 10. It makes planning life difficult because you have no idea how much you'll earn each week.

Managers use it to bully, companies use it to exploit staff, and workers are usually too desperate to dare turn it around and use it to their advantage. The contract goes both ways. If the employer wants you to work and you have prior commitments you can usually refuse, but most workers fear being denied hours in future if they stand up for themselves so instead they get shafted, as usual, by business interests.

They're an awful tool of exploitation that only suits a very small subset of people who wish for extreme flexibility in their work, but are increasingly being used as standard employment contracts by companies wishing to control and manipulate staff.

They have a legitimate role, but 9 times out of 10 that's not what they're used for.