A group of tissues working together as a unit for a particular purpose in the body?
Answer Is an Organ. Such as the Heart or Lungs. Is an organ system.
First off, a group of similar cells working together is a tissue, and a group of tissues working together is an organ. Organs are then put together to make systems e.g. circulatory sistem includes the heart, respiratory system involves the lungs and so on. Similarities can be that they all origionate with a cell or a group of cells, they all have a purpose in that they contribute to a greater system.