There is no difference. They are different names for the same thing: pressure ulcer, bedsore, and decubitus ulcer all describe one kind of wound.
The definition is simple. A pressure sore is an area of the skin that breaks down when something keeps rubbing or pressing against the skin.
A fourth term, pressure injury, is used in the staging language. It’s used for a deep tissue pressure injury, and when talking about a pressure ulcer forming. Patients will meet all four, often in the same conversation.
Knowing they are one thing matters for a practical reason. Searching for one term will miss information written under another, and a note that says decubitus ulcer and a leaflet that says bedsore are describing the same wound.
What matters more than the name is the cause. Pressure sores occur when there is too much pressure on the skin for too long. This reduces blood flow to the area, and without enough blood to nourish the skin, the skin can die and a sore may form.
The word bedsore is the one that misleads most, because it suggests a bed. Using a wheelchair, as well as staying in bed for a long time, makes a pressure sore more likely. Each has its own repositioning guide: about every 2 hours in bed, and every 15 minutes in a wheelchair.