Cells group together to form tissues, which then group together to form organs and organ systems. There are four main types of tissues - epithelial, connective, muscular, and nervous tissue. Epithelial tissue covers surfaces and lines organs/cavities. It protects and absorbs. Connective tissue provides structure, protection, insulation and transport. It includes bone, cartilage and fluid connective tissues. Muscular tissue includes three main types that allow movement. Nervous tissue is made of neurons that receive and transmit nerve impulses.