A tissue is a group of cells with a shared structure and job. An organ brings several tissues together to carry out a larger function. A leaf, for example, contains different tissues involved in collecting light, exchanging gases and moving water. The leaf works because those parts act together.
Biologists use levels such as cell, tissue, organ and organ system to describe organisation. The levels are a way to ask which scale explains a question; naming a level does not tell the full story of how its parts interact.