Bikes designed for dirt roads (hence the name) and singletrack as well as pavement. These bikes have a similar appearance to road bikes but tend to have longer wheelbases, more upright posture, lower bottom brackets (for stability), and clearance for wider tires.
Also called "adventure bikes" or "all-road bikes" these are designed to work on broken (but mostly compacted) surfaces as well as on roads. They are based largely on road bikes (including such features as drop bars) but with a longer wheelbase (and sometimes a lower bottom bracket) for increased stability, clearance for wider tires and a more upright riding posture. They differ from cyclocross bikes, which are more optimized for maneouverability, and mountain bikes, which will have suspension and more ground clearance.