Cells are the basic building blocks of life, which can join together to form tissues, while different groups of tissues join together to form organs. These organs are then connected together to form organ systems, such as the digestive system, which includes the stomach, esophagus, intestines and other organs.
Tissues are made up of groups of cells that all have a similar function and structure. Some examples of tissues include muscles, bones, skin and the lining of the stomach, lungs and intestines. The lining of the stomach is just one of the many tissues that have joined together to form the organ, as it also contains muscles, mucus membrane tissue and many other tissue types.
Different species of plants and animals have different types of cells and thus different types of tissues, organs and systems. In humans and other animals, there are four main types of tissues: connective, muscle, nerve and epithelial. Each of these groups can be further broken down, as there are a number of different tissues within each family. For instance, the human body contains three different types of muscle tissue and two types of nerve tissue. Furthermore, blood, bones and cartilage are all types of connective tissue.