Cells organize into tissues, which then organize into organs that work together in organ systems. There are four main tissue types - epithelial, connective, muscle and nerve tissue. Epithelial tissues line body surfaces and internal cavities. Connective tissues bind other tissues together and provide structural support. Muscle tissues contract to cause movement. Nerve tissues transmit electrical signals to coordinate body functions. Organs contain multiple tissue types that work together to perform specific functions, and organ systems group related organs to carry out major processes like digestion and respiration.