Ghusl literally means "bath". In cetain circumstances Muslims are required to bathe completely with clean water as a means of purification. This ritual bath in Islam is called "Ghusl". The circumstances where ghusl is required include (but is not limited to):
Ghusl for menstruation
. After ejaculation (through sexual intercourse or otherwise)
. Upon coming in contact with a dead body
Tayammum is the Islamic term for "dry ablution". This is the type of ablution performed when clean water is not readily available, or it is not feasible for a person to perform regular ghusl or wudhu because of medical reasons or similar issues. In such circumstances, instead of water, the person wipes a small amount of sand, dust or dry earth on his/her face and hands.
source : sibtayn.com
