Touring bikes are a sub-classification of road bikes.  These are the “long-haul truckers” of the road world.  Touring bikes can cross the continent, over rough roads and inhospitable conditions, with little complaint.

Touring bikes have to be absolutely bomb-proof as far as durability and reliability.  Purists love all-steel bikes (frame and fork), but aluminum and titanium touring bikes are becoming more common.  Although touring and road geometry are basically the same, a touring bike is heavier than a road bike and built for durability.  At the same time, it’s lighter than a mountain bike and the larger road-sized wheels provide less rolling resistance.  Wheels are very sturdy - some riders use tandem wheels, or at least wheels with a very high spoke count.

A touring bike has a triple chainring and some even have mountain-bike gearing in the rear (mountain cassette/rear derailleur).  Even strong riders fatigue easily on hills when hauling panniers full of gear!  Components must be absolutely dependable and shifters are typically are not high-end race-worthy but rather mid-grade, workhorse shifters.  To be stuck in the middle of nowhere with a broken shifter is not fun!

A touring bike should have enough eyelets and braze-ons to attach anything a rider may need on a cross-country journey:  racks, fenders, panniers, lights, extra water bottle cages, trailers, etc.

Touring bikes accommodate wider tires (for durability and long-ride comfort, from 35c to 45c.  Tires are quite heavy and puncture-resistant.  Many riders will add liners to their tires to further improve puncture resistance.