Wallet checks are a compact personal check format, sized to fold and fit into a standard wallet — smaller than a typical personal check, which itself is already smaller than a business check.
Why some people prefer them
They're a convenience format rather than a different type of check functionally — the same routing number, account number, and signature requirements apply. The appeal is simply not needing a separate checkbook or check register binder; the smaller checks fit directly alongside cash and cards.
If you print your own checks rather than ordering pre-printed wallet checks, you'll need a check layout sized specifically for this smaller format — printing a standard personal check template onto wallet-size stock will run off the edges.