Human organs are made of tissues, and tissues are made of cells. An organ is a structural unit of several tissue types joined to serve a common function; the intestinal wall, for example, combines epithelial tissue and smooth muscle tissue.
The four basic tissue types in the body are epithelial, connective, muscle, and nervous tissue. Organs that work together, such as the stomach and intestines, form an organ system.
Source: Organ (biology), Wikipedia
Image: Internal organs by Mikael Häggström, public domain, via Wikimedia Commons.