Groups of very similar of cells gathered together to perform a specific function are referred to as a tissue. Our bodies have many tissues. Perhaps the one most familiar to us is the epidermis. The epidermis is the tissue that forms the surface of our skin.
Some other tissues you might be familiar with are skeletal muscle, nervous tissue, mucosal tissue (mouth and nose) and bone tissue. You might not be familiar with other types of muscle like smooth muscle and cardiac muscle. There are many other types of tissues most of us only hear about if we study medicine or have a problem with that tissue.
Different tissues heal at different rates. For instance, skin and mucosal (mouth) tissue heal quickly. Injuries to these heal in a few days to weeks. Bone tissue heals more slowly. Bone injuries take weeks to months to heal. Other tissues like muscle heal more inconsistently, sometimes talking days, sometimes months or even not at all. A few tissues, usually highly specialized ones do not heal at all. Good examples of tissues with minimal to no ability to heal are cardiac muscle and central nervous (brain) tissue. One of the main motivations behind adult stem cell research is to find effective ways of obtaining healing in these poorly healing tissues.