There are four main categories of tissue: connective tissue, epithelial tissue, nervous tissue, and muscle tissue. Connective tissue is classified by loosely associated cells in an abundant medium, generally used to fill spaces and connect structures to one another within the body. Collagen fibers are particularly abundant in connective tissue. Adipose cells (fat), blood, cartilage, and bone are all specific types of connective tissue. Extracellular matrix is generally also considered connective tissue, but can vary in composition to some extent by location and includes loose connective tissue and dense connective tissue.

In contrast, epithelial tissue serves to line and protect regions of the body form the external environment. Any region of the body that is directly or indirectly connected to the environment is lined with epithelial cells, including the skin, digestive tract, respiratory tract, and excretory tract. Since material from the external environment (food) can enter the stomach, we know that the stomach lining must be made of epithelium.

Connective tissue supports and connects other tissues and parts of the body. Connective tissue includes bone, cartilage, and fat. Epithelial cells form the epidermis of the skin and the linings of the digestive and respiratory tracts. Canaliculi are channels that allow osteocytes (bone cells) to remain in contact and transmit nutrients and gasses. Proteins are essential for the growth of new tissue or the repair of damaged tissues. Minerals are generally inorganic, but the mineral hydroxyapatite forms the matrix of bones.

Skin is part of the integumentary systemand is composed of stratified squamous epithelium.

Connective tissue is defined as a population fo cells suspended in an extensive extracellular matrix. For example, blood is classified as connective tissue because it consists of red blood cells (erythrocytes) suspended in a liquid matrix (plasma). Bone, cartilage, ligaments, and adipose tissue (fat) are also considered connective tissue.

Connective tissue supports and connects other tissues and parts of the body. Connective tissue includes adipose (fat), cartilage, bone, and blood. Muscles produce movement. Muscles are attached to bone, in the walls of hollow internal organs, and in the heart. Nervous tissue transmits impulses in from sensory receptors and out to produce motor responses. Types of nervous tissue include the brain, spinal cord, and nerves. Epithelial tissue protects, secretes, absorbs, and excretes. Examples of epithelial tissues include skin, glands, and mucous membranes. In dentistry, the term "hard tissue" is used to describe the calcified tissue components of the tooth: enamel, dentin, and cementum.

Connective tissue is used to support, protect, and transport within the body. Bone tissue forms skeletons. Cartilage connects and supports bones, and adipose tissue stores energy and provides cushioning. Epithelial tissue refers to another type of tissue.

Adipose tissue is one of the main types of connective tissue.  It is also known as fat, and is a loose connective tissue composed mostly of adipocytes.  

Cartilage is a flexible connective tissue found throughout the body in places such as the ear, nose, bronchial tubes, and intervertebral discs. It is not as hard as bone, but it is stiffer and less flexible than muscle. Cartilage is composed of specialized cells named chondrocytes. These cells produce and maintain the cartilaginous matrix, which is composed of collagen and proteoglycan.