Bean is the name used for the seeds of several plants. Most of these plants are known as legumes. Their scientific name is Fabaceae. Common beans, lentils, soybeans, peas are some types of beans. Beans contain a lot of fibre. They are also high in protein. Vetches and lupins are all in that group of plants but not all legumes are edible. Laburnums for example are very poisonous as are some lupins.
The seeds of other plants (which are not Fabaceae), are sometimes also called beans. This is mostly because look like true beans. Example for such beans are Coffee beans, Cocoa beans, and Vanilla beans.