are a special type of road bike designed to be ridden on mixed surfaces (combination of pavement, unpaved trails, gravel, grass). They have a drop handlebar like regular road bikes, but the tires are a littler wider and have aggressive tread for more off-road traction, and they have a different style of brake that helps to prevent mud buildup in the frame.
Cyclocross bikes make good commuter bikes, because of the versatility, durability, and in most cases, the ability to carry light loads.
Cyclocross bicycles are sometimes called 'cross bikes or cx bikes' for short.
Gravel Bicycles
are optimized for comfort over longer distances ridden on gravel and fire roads and sometimes even some smoother singletrack. Gravel bikes tend to have more mountain bike-like geometry. They come with tire clearances of at least 40 mm. That means you can run relatively wider tires without fear of them rubbing against your frame or fork. Bigger tires mean more comfort while pedaling hour after hour over rugged gravel terrain.