What it is
A column wheel is a tiny stepped wheel with 6 or 8 columns (or teeth) projecting upward from its rim. As the wheel rotates one position with each pusher press, the columns either engage or disengage levers that control the chronograph's clutch, reset hammer, and power-reserve. The wheel acts as a mechanical state machine: each column position represents a chronograph state (off, running, reset). Cleanly designed, smoothly operated, and historically the haute-horlogerie way to build a chronograph.
How it differs from cam-and-lever
Cam-and-lever chronographs (Valjoux 7750, ETA 7750-derived) use a flat cam with shaped grooves; levers ride along the cam and lift up or drop down based on the cam profile. The mechanism is reliable, easy to manufacture, and works fine, but the pusher action has a distinct 'click' when the lever drops off a cam edge; engagement is less smooth. Column-wheel chronographs feel smoother to operate; the columns provide gradual lever movement rather than abrupt engagement.
Where to find them
Modern column-wheel chronographs: Rolex Daytona Cal. 4130, Patek CH 29-535 PS, Audemars Piguet Cal. 4400, Zenith El Primero, TAG Heuer Heuer 02, Breitling B01, Omega Cal. 9300/9900. Cam-and-lever: most CHF 2,000-8,000 chronographs (Valjoux 7750 family).
What it costs
Column-wheel chronograph movements typically cost 30-100% more to manufacture than cam-and-lever equivalents. The retail-price differential between a column-wheel and a 7750-based chronograph at the same brand: roughly 2-4x. The differentiator on a chronograph spec sheet: 'column wheel' is shorthand for 'this is the premium chronograph tier'. See wiki: column wheel.