Ghusl is to wash the whole body (water touches the whole body). Preferably in a recommended way (following sunnah).
Wudu is to wash only some parts of the body : hands, face, head, feet. In a ceratain order.
A muslim can do prayer
only if he is in a state of
valid ghusl and wudu. You can do prayer as long as you don't break your last ghusl and your last wudu. If you break your wudu or ghusl, you should redo it before the next prayer. Ghusl can replace wudu (if you do ghusl, wudu is unnecessary).
Things that invalidate/require ghusl :
- After the discharge of semen as a result of stimulation whether awake or asleep (i.e. in a wet dream).
- After sexual intercourse performing Ghusl is required.
- Following a period of menstruation, a woman is required to perform Ghusl.
- At the end of 40 days after childbirth, a woman is required to perform Ghusl but if the bleeding stops before completing the 40 days period, then she must make the Ghusl as soon as the bleeding stops.
- When a Muslim dies, Ghusl is required for him/her except for the Jihad Martyr (Shaheed).
Things that invalidate/require wudu :
1. Anything coming out as urine, feaces or gas etc.
2. A person falling sound Sleep
3. Becoming Unconscious - insanity or fainting or intoxication
4. Touching the Uncovered Private Parts
(A person can break his wudu without breaking his ghusl)
Further information
GHUSL : http://www.muslimconverts.com/prayer/ghusl.htm
WUDU : http://www.muslimconverts.com/prayer/how-to-perform-wudhu.htm