A coach bolt (also 'carriage bolt' in US usage) has a smooth domed head with no driver socket. Under the head, the shank is squared off for the first 10 to 20 mm. As the nut is tightened on the threaded end, the square shank bites into the timber under the head and stops the bolt from spinning.
Coach bolts were originally for fixing wagon and coach hardware to timber wheel hubs and frames. In gate work today they're standard for fixing steel hinge straps or strap-hinge plates to timber gate posts and timber rails. The clean domed head is also vandal-resistant: there's no driver socket to put a tool in.
Common Australian sizes are M8, M10, and M12 in galvanised mild steel.