A group of cells that work together and form an organ is called tissue, or tissues in the plural. Tissues occupy the level of biological organization between individual cells and organs.
The cells in a tissue usually share a similar structure, developmental origin, and specialized job. In animals, the four basic tissue types are epithelial, connective, muscle, and nervous tissue. The stomach, for example, contains several tissue types working together, including muscle tissue for movement and epithelial tissue for lining and protection.
An organ is not usually made from just one tissue. It is a coordinated structure containing multiple tissues, while organs that cooperate form an organ system. In plants, tissues include xylem and phloem, which transport water, minerals, and sugars.
The scientific study of tissues is called histology. French anatomist Xavier Bichat helped establish the concept around 1801, before scientists had modern microscopes.