Tissue is a group of cells having similar origin, structure& function. They are integrated group of similar cells that perform a common function and combine to form organs.Study of tissues is called Histology.
In unicellular organism (Amoeba) single cell performs all basic functions, whereas in multi-cellular organisms (Plants and Animals) shows division of labor as Plant tissue & Animal tissues.
Animals have four main categories of tissues:
1. epithelial tissue,
2. connective tissue,
3. muscle tissue, and
4. nervous tissue