A tissue is a categorical level of organization on the cellular level within biological organisms. A tissue is a grouping of cells who chiefly possess similar embryonic developmental origins as well as similar functions. A grouping of these tissues is then known as an organ.
There are over 200 distinct animal tissues with separate origin and function, such as adipose tissues, cartilage, and spinal tissues. However, they can be generally categorized into four types: epithelial, connective, muscle, and nervous tissues.
Biological tissues are studied via a branch of microbiology known as histology.