Bedsores is a pressure ulcers which occur over the bony prominences of the body due to pressure over the area for prolonged period or due to shearing or friction. Usually seen on bed ridden patients.
Causes
There are mainly three different factors which may lead to bedsores. These are 
1) Pressure: sometimes compression of the tissue adjacent to bony prominences or compression of the muscles may be the most important underlying cause of bedsores. 
2) Shearing: the force of shearing is also an important factor which contributes to the formation of bedsores.
3) Microclimate:  Bed sore is caused due to temperature and the moisture of the skin that are in contact with wheelchair, bed, etc.  
Risk factors
There are several risk factors associated with bedsores. The most common one being inability to move around; thus maintenance of same posture for prolonged period. This type of immobility occurs due to the poor general health condition of the person, paralysis, during recovery period following surgery, sedation and coma.
There are several other risk factors associated with bedsores; these are
1) Age: age of a person is an important determinant for bedsores. For example elderly people have less elastic and much drier skin in comparison to younger people and it is more susceptible to damage. Again elderly people have slower rate of cell growth so healing of even a superficial ulcer is much delayed in elderly people.
2) Inability to feel pain: inability to feel pain and temperature fluctuation may lead to trauma to the legs and thus increased chance of bedsore formation. Especially people with nerve related diseases like spinal cord injury in patients are at increased risk.
3) Weight loss: it leads to loss of the cushioning fatty tissue between the bones and the skin leading to increased chance of injury to the skin
4) Lack of proper nutrition: balanced diet is very important for good health of a person
5) Presence of excess moisture in the air or complete lack of it may increase the chance of skin damage due to frictional force.
6) Inadequate control of bowl movement may lead to leakage of stool and contact of the skin with fecal matter for prolonged period of time. The bacteria present in the fecal matter also help in infection spread.
7) Certain other medical problems which hamper blood flow through vessels may increase the incidence of bedsore.
8) Smoking tends to lower blood supply thereby also making the smokers susceptible to tissue injury.
9) Muscle spasm and purposeless involuntary movement of muscles may also increases the chance of tissue injury and thereby bed sore production
10) Poor alertness increases the chance of suffering from injury leading to increased incidence of bedsores.
Treatment 
Usually stage I and II heal with time without much medical attention within weeks to months however stage III and IV are difficult to treat.Before addressing the issue of treatment a team of experts consisting of general physician, physician expert in wound management, nurse, social workers, physiotherapist, dietician, neurosurgeon, orthopedic surgeon etc should be consulted.The main factor causing bedsore is pressure. Thus reducing pressure by repositioning and use of support surfaceWound dressing is to be done for regular cleaning and dressing of the wound. Removal of excess protein can be helpful in established cases of bedsore. It can be done through surgical debridement, mechanical debridement, autolytic and enzymatic debridement. Negative pressure therapy  may also help in would sealing.Other associated treatment options include pain management, healthy and balance diet consuming, antibiotic drugs to fight infection, management of incontinence, relief of muscle spasms by using muscle relaxants etc. If all the above approaches of treatment fail surgery is only treatment option.