Passing lane

A passing lane (North American English) or overtaking lane (English outside of North America) is a lane on a multi-lane highway or motorway closest to the median of the road (the central reservation). In some countries, lanes are described as being on the 'inside' or the 'outside' of a road, and the location of the passing lanes will vary.
In modern traffic planning, passing lanes on freeways are usually designed for through/express traffic, while the lanes furthest from the median of the road have entry/exit ramps. However due to routing constraints, some freeways may have ramps exiting from the passing lane; these are known as "left exits" in North America.
A passing lane is commonly referred to as a "fast lane" because it is often used for extended periods of time for through traffic or fast traffic. In theory, a passing lane should be used only for passing, thus allowing, even on a road with only two lanes in each direction, motorists to travel at their own pace.
A 2+1 road has a passing lane only in one direction, usually alternating each few kilometers or miles. In practice, they are more like upgraded highways than motorways.
Misuse and common practice in the United States.
Common practice and most law on United States highways is that the left lane is reserved for passing and faster moving traffic, and that traffic using the left lane must yield to traffic wishing to overtake.
The United States Uniform Vehicle Code states:
Upon all roadways any vehicle proceeding at less than the normal speed of traffic at the time and place and under the conditions then existing shall be driven in the right-hand lane then available for traffic ...
It is also illegal in many states in the US to use the "far left" or passing lane on a major highway as a traveling lane (as opposed to passing), or to fail to yield to faster moving traffic that is attempting to overtake in that lane. For example, Colorado's "Left Lane Law" states:
A person shall not drive a motor vehicle in the passing lane of a highway if the speed-limit is sixty-five miles per hour
or more unless such person is passing other motor-vehicles that are in a non-passing lane...
In some states, such as Maine, Massachusetts, Missouri, Montana, New Jersey, and Washington, the center lanes on highways with three or more lanes in each direction are also passing lanes. In those states, no matter how many lanes there are on the highway, drivers must not leave the right-hand lane unless they are overtaking slower vehicles, making a left turn or exit, or obeying a move over law. 
In California, at least, there is no prohibition against cruising in the left lane. However, similar to the states below, slow traffic must stay right. 
In other states, such as Massachusetts, New Jersey, Illinois, Pennsylvania, and others, it is illegal to fail to yield to traffic that seeks to overtake in the left lane, or to create any other "obstruction" in the passing lane that hinders the flow of traffic. As a result, heavy trucks are often prohibited from using the passing lane.
The left lane is commonly referred to as the "fast lane", but that is not an accurate description of the lane's purpose. The left lane is the designated passing lane, however, vehicles in the left lane must obey the posted speed limits. A common problem arising from misuse of the left lane is speeding and tailgating. These actions create road rage and increase overall danger.
A driver hoping to pass a slow motorist in the "fast lane" can be stuck in an awkward situation. One strategy is to signal a lane change toward the center median. Another is to flash headlights. A third, which is dangerous and illegal, is to drive very close to the "fast lane" driver's bumper (this is known as tailgating).
Most commonly, motorists will attempt to overtake the outer car on the inner lane either to continue at a fast pace or to pass a car that is going too slowly in the passing lane. On high-capacity multilane freeways (three or more lanes per direction), many motorists often pass on the inner lane, largely in response to misuse of the "passing lane" by slower traffic.
In some areas, such as the U.S. states of Colorado and Kentucky, vehicles in the left lane are required to yield to faster traffic only if the speed limit is above 65 miles per hour. In other areas, like Alaska, there is no law requiring slower traffic to move over for faster traffic.
Signage.
The use of passing lanes for faster traffic is sometimes acknowledged with signs using phrases such as "Slower Traffic Keep Right" (in Canada, where the passing lane is to the left). In a study by the AASHTO Subcommittee on Traffic Engineering, all 24 U.S. states involved used some form of passing lane courtesy signage, 9 of which only use those signs for steep graded roads.
Proper use.
Many areas which make it illegal to fail to yield to faster traffic also have exceptions to those rules. Some of these exceptions include preparing to make a left turn, taking an exit located on the left side of the roadway, avoiding traffic merging onto the roadway, or overtaking and passing another vehicle.
In the province of Quebec, it is illegal to travel in the left lane when not passing when the speed limit is over 80 km/h (50 mph).
In the province of Ontario, unless otherwise posted, the left lane on a multi-lane highway is not reserved for passing or to give motorists a special lane to violate the speed limit. Only vehicles traveling less than the posted speed limit are required to stay in the right most lane. Passing on the right is perfectly legal in Ontario on multi-lane highways.
Truckers often use the passing lane in moderate traffic where legal to do so to reduce travel times, however in many areas, tractor trailers are banned from using the passing lane for safety reasons; these restrictions are normally found along urban, often congested highways with multiple lanes (e.g. Interstate 40 west of Raleigh, North Carolina), or on rural freeways with 6 or more lanes (3 in each direction).
Hammer lane.
The hammer lane is another term for the passing lane. Its etymology originated with truckers in North America in reference to slamming the accelerator with a foot like hammer.) 
HOV lanes are not usually considered hammer lanes, but are also used for express travel by commuters.
Climbing lane.
In hilly terrain, some undivided highways are built with three lanes, with the extra added pavement known as the "climbing lane" or "crawler lane". Two lanes are used for traffic heading in the uphill direction, with one lane being a passing or climbing lane, and one lane is used for downhill traffic. On dual carriageways, the climbing lane may be marked with a broken double white line.
Australian use.
In Australia, most intercity highways are constructed with only one lane in each direction. Head-on collisions are a risk, particularly with fatigued drivers. Overtaking lanes are an additional lane in one direction for a short distance (one or two kilometres) to assist faster traffic to safely overtake slower traffic. Road markings are often painted so that the additional lane appears in the centre of the road (guiding traffic to the left), and traffic needs to deliberately change lanes to overtake. At the end, the markings are the other way, so that the left lane must yield and merge into the overtaking lane. Large signs alert drivers that they are approaching an overtaking lane, often at 5 km and 1 km distances before the lane starts.