In the course of a flight, it may become necessary for the flight crew to carry out an unplanned landing due to situations such as aircraft mechanical failure or passenger medical emergencies. There are numerous types of emergency landings.
Precautionary landings are by far the most commonplace. These can result from indications that any one of several of the aircrafts flight systems (electrical, pneumatic, engines etc) are not performing entirely without fault. There is usually no threat to the safety of all those aboard in these events and weather to divert or not for a precautionary landing is subject to discretion, rather than being forced to.
A forced landing takes place when factors beyond the control of the flight crew conspire. Major system failures on board the aircraft which can severely limit the flight crew's ability to maneuver the aircraft and endanger the safety of passengers is one such occurrence. If the weather at a destination is below the minimal for a safe landing, the flight may be diverted to another airport, this too, is a forced landing. If an aircraft violates the airspace of any sovereign nation without correctly identifying itself and its intentions, it may too be forced down by military action.
However, not all emergencies are so fortunate as to be able to enjoy the privilege of a runway. Crash landings are landings that are not carried out on a runway. Engine or flight control failure may totally hinder the ability of the flight crew to maneuver the aircraft to an airport. A forced landing must be carried out in the terrain they are flying over. The most suitable area for landing is chosen and the pilots attempt to land the aircraft there with the least number of injuries to passengers. Emergency landings carried out on water are also known as Ditching. The survivability rates of emergency landings are usually pretty good when it comes to those preformed on land. Water ditching are a bit lower, albeit not unsurvivable.
The likelihood that a passenger will be involved in any emergency landing, let alone a forced one, is extremely rare. Modern airliners have multiple redundant systems that can take several consecutive failures and still function. Pilot training in simulators to deal with these events and good maintenance have even further made such events rare. There is however one variable that man cannot minimize or control. That is the weather. Diversions due to weather phenomena aren't particularly rare, but they are harmless, although a bit inconveniencing.