The traditional touring bike usually has the following distinctive features:
Drinking bottles
It is very important to stay hydrated whilst undertaking any exercise. Fluids are lost through perspiration, and when cycling, the breeze can often mask the amount of fluid lost. It is important to carry sufficient fluids on the bike, and top up regularly.
Triple chainset
The three chain rings give a wide range of gears which allows the rider to climbs hills even when fully loaded with panniers.
Front rack
This front rack allows front panniers to be located where they have a low centre of gravity and less effect on the steering of the bike. The addition of front panniers can also serve to balance the weight distribution on the bike instead of it all being at the rear. A bar bag is usually added which will carry a map, wallet, phone and camera
Rear carrier
A sturdy rear carrier is needed for loaded panniers. This one has a 4-point fixing with special braze-ons on the frame. This rack can be used with panniers as well as a rack-bag, often seen used by day cyclists.
Don't think about touring anywhere wet or muddy without mudguards. They prevent so much muck being sprayed all over you and your bike.
Cycle computer
Now an inexpensive device, and a real must for any cyclist. It assists in navigation in that you can compare distances travelled accurately with your map, and your average speed will allow you to calculate how long a journey should take.
Most touring bikes are made of alloys of steel. Steel has excellent vibration absorption properties. Top end bikes are actually light and responsive as they use very high tensile, thin walled steel tubes. Steel frame bikes have a reputation for longevity. Popular tubeset manufacturers include Columbus, Vitus and Reynolds.
More recently, Titanium has been used for touring bikes. Lightweight, compliant, robust and corrosion resistant, it is perhaps the ultimate material... except for the price.
Touring bikes need strong wheels to cope with additional weight, rough tracks, and give longevity. They are normally made with wider rims than a racing bike to cope with tyres from 28mm - 38mm width. Many spokes (32/36/40) mean that the load per spoke is reduced and will give many thousands of miles of trouble-free riding.
The most common manufacturer is Shimano, but Campagnolo now make some quality components for touring bikes. Most new bikes come with 24 or 27 gears, but bikes with 30 gears are slowly coming into the marketplace.
Shimano have had many years experience making gears for mountain bikes, which need to be robust and reliable. Many of these components are used on touring bikes for the same reasons.
Both systems use a method of changing gear built into the brake levers (on dropped handlebars) and are far safer than previous methods
Bob Jackson  Mercian  Thorn Cycles  Dawes Cycles