Adventure bicycles are arguably the most versatile bikes on the planet, while they come in a variety of forms they are all designed to be equally capable on a long on-road tour as on a day getting away from it all on dirt roads as on some light single track riding. They stemmed from cyclocross bikes which look like road bikes but are built for riding off pavement. Today there is a huge variety of adventure bikes on the market all catering to different ideal rides, but at the end of the day, versatility is the key for these machines.
It really can be tough to draw a through-line between every adventure bike on the market but for the most part they’ll have the drop handlebars (to provide a variety of riding positions) and 700c wheels (to make riding efficient, fast, and fun) of cyclocross bikes. Nearly all of them will come with a variety of cargo rack connection points, like classic touring bikes, so they can be laden with gear for long tours or bikepacking. However, they typically won’t have the super slick tires of a road bike (again for gravel riding) or the suspension of a full-fledged mountain bike.
For a great many people this category will hold your ideal bike, and especially those looking for a “one bike quiver” that allows them to do all of their riding on one bike these are a great fit. However, they will not be as efficient on pavement as true road bikes (or even many urban/ hybrid bikes), and they won’t have the off-road capabilities of a dedicated mountain bike.
While many adventure bikes come with road bike standard 700c (28ish inch) wheels you will see options that run from small 26” wheels to large 29” wheels, they all have their own advantages and disadvantages. You may even see the same bike available with different wheel sizes (looking at you Surly Disc Trucker). Smaller wheels typically are sturdier and behave better under load, hence why many touring/ bikepacking oriented rides will have them. While larger wheels roll more efficiently both on and off-road, making them ride faster and smoother.