So many things. A gravel bike is a beautiful and weird hybrid of road bike, adventure bike, and cyclocross bike with just a hint of mountain bike thrown in. In terms of geometry and ride feel, a gravel bike is very similar to a road bike.

Whereas a cyclocross bike will have a shorter wheelbase for maneuverability on slow moving, technical cross courses, gravel bikes feature a slightly longer wheelbase for excellent stability at speed on the road or on gravel. The result is a super capable bike for gravel racing, bike packing, adventuring, or big days in the saddle.

The single biggest difference between a gravel bike and a road bike is tire clearance—gravel bikes feature tons of clearance for wide, grippy tires. Gravel bikes will also feature multiple contact points for bike bags, racks, or fenders. You’ll get a lot of the benefits of a road bike, for sure—efficiency on climbs and flats, excellent stability at speed, and good handling manners.

But you also get a tougher overall package able to handle a huge range of terrain. Many gravel bikes will also be 650b wheelset compatible—a good upgrade for anyone looking to tackle especially rough or difficult gravel roads or terrain. 

Definitely! Well, within reason. Gravel bikes offer a fun challenge on flowy singletrack with bumpy sections—especially if you run a tubeless tire setup (less likelihood of pinch flats). Tackling big gravity trails or super burly terrain, however, is a less good idea. A gravel bike will definitely not survive the bike park or enduro sections, but it can hold up and be a lot of fun on mellower cross country trails. 

So when weighing gravel bike vs mountain, think about what you might want to be doing. A mountain bike will always be able to ride more trails than a gravel bike. So if trails get you excited, buy a cross country, trail, or enduro mountain bike. But if the idea of riding efficiently on the road, gravel paths, logging roads, and the odd mellow mountain bike trail appeals to you, go for gravel.