بسم الله الرحمن الرحيم
Ghusl - full-body washing
Ghusl means washing your whole body with water once. It is a ritual purification that removes the state of major impurity. Being in the state of purity enables one to perform prayers, recite the Quran, and visit a mosque.
Ghusl has three obligatory actions
Wash the entire body.
Rinse the entire mouth.
Rinse the nose up to the bone.
Performing ghusl once keeps you in the state of purity until something an event happens that invalidates it.
When is it obligatory to perform ghusl?
One must perform ghusl when any one of the following three things occur:
Orgasm - the emission of sperm/sexual fluid;
Sexual intercourse, even if without an orgasm;
The ending of menstruation or post-natal bleeding;
Any of the three actions above enter one in the state of major ritual impurity. While ghusl removes this state, and makes one pure again.
Additional Commentary
Becoming a Muslim: ghusl is also performed after one takes the shahada and becomes a Muslim, if any of the above occurred before accepting Islam.
Friday Prayer: it is sunnah of the Beloved Prophet (upon him be blessings and peace) to perform ghusl before the Jum'ah prayer and the two Eid prayers.
What can't be done without ghusl?
Without ghusl one can't do any of the following:
Perform ritual prayer (namaz)
Enter a mosque
Touch the Quran or separate ayahs
Recite the Quran (even from memory)
Perform tawaf around the Holy Ka'ba.
Women's special states
During the menstruation and post-natal bleeding women do not perform ritual prayers and do not fast. Such missed obligatory fasting days (of Ramadan) have to be compensated afterwards, i.e. she observes the missed fasts later (before the next Ramadan comes). The prayers, in contrasts, do not have to be compensated. Importantly, the rulings related to menstruation and post-natal bleeding are well-defined in Islam, however mostly omitted in this app for the sake of brevity. It is obligatory for sisters to learn them.
How to perform ghusl according to sunnah?
According to the sunnah of our beloved Prophet (peace and blessing be upon him) the ghusl is performed in the following order:
Intention (niyyah) followed by saying "Bismillahir-Rahmanir-Rahim" (before revealing one's nakedness).
Washing hands until the wrists.
Washing one's private parts (front and rear).
Washing of any filth from the body.
Rinsing one's mouth and nose properly.
Performing wudu the usual way (though washing one's feet can be delayed until the end of ghusl).
Pouring water and washing one's entire body starting with the head for a total of three times.
One should avoid unnecessarily wasting water or speaking. It is prohibited to reveal one's nakedness (private parts) in public places (e.g. gym or swimming pool).
Ghusl while fasting
A fasting person must not let any water down his throat, thus that person should exercise caution when rinsing his mouth and nose.