Plain bearing would be best described as mechanical components with bearing. It enables a sliding or rotational motion between two parts of the machine. The major purpose would be preventing wear and tear of the various points of support of a machine. A machine would suffer wear and tear when the shaft is rolled directly on the structure.
If you were involved in a project without having knowledge about the kind of bearings you require, rest assured you would have a big trouble. Rest assured that plain bearings would be composed of two parts. The first part would bear the load, called the support. The second part would be the area that is worn in contact with the moving element – the bush.