There are no strict rules on what makes a good touring bike. Some people like to ride drop handle bar bikes others will use a mountain bike position with flat bars however there are a few things which all bikes suitable for touring will have in common.
Whether you are planning a weekend's jaunt or a long-term epic you need space to carry your essential belongings. The best way to do this is in panniers and on a luggage rack. This allows your bike to carry the load and remain well balanced to maintain its handling.
A touring bike will come with rack mounts front and back; it will be well built and sturdy to carry the additional loads. Some adjustment is made to geometry to allow a more upright position for rider comfort during long days in the saddle and to help maintain the handling of the bike when fully loaded.
Gearing will need to be lower than on the standard bike to help you manage gradients when carrying additional weight, most people favour a triple chain set or low ratio mountain bike gears.
Brakes need to be powerful, as they will be stopping a lot more weight than usual.
Wider tyres with extra puncture resistance will help soften the ride and cope with the extra load on your bike. Mudguards are also a useful feature, as it will protect your belongings on the luggage rack being covered with water.
Comfort on the bike is paramount; position is the most important part of this so make sure your bike is correctly fitted. Next make sure that you have the right saddle for you, do some long rides on it before heading off for your first tour. This will help break it in and ensure that you can be comfortable on it for several hours at a time.
You may want to have padding under your bar tape or soft grips to help ease pressure on the hands if you are planning long ride every day of your tour.