Shaykh Ibn 'Uthaymeen explained that the distinction between wudu (ablution) and ghusl (full bath) is based on the type of ritual impurity one needs to remove.

Wudu is prescribed for cleansing from minor ritual impurity (al-hadath al-asghar). This state of impurity results from actions such as passing urine, stool, or wind; deep sleep that causes loss of awareness; or touching the private parts directly without a barrier. Wudu involves washing specific parts of the body: the face, the hands and forearms up to the elbows, wiping the head, and washing the feet up to the ankles.

Ghusl, on the other hand, is obligatory for cleansing from major ritual impurity (al-hadath al-akbar). This state, also known as janabah, arises from sexual intercourse, ejaculation (of semen), or the cessation of menstruation or post-natal bleeding for women. Ghusl involves washing the entire body with the intention of purification, ensuring that water reaches every part, including the roots of the hair.

The primary evidence establishing this difference is in the Qur'an, where Allah says:

"O you who have believed, when you rise to [perform] prayer, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles. And if you are in a state of janabah, then purify yourselves..." ([Surah al-Ma'idah 5:6](/quran/5/6)).

Shaykh Ibn 'Uthaymeen clarified that this verse outlines the actions of wudu for the state of minor impurity and commands ghusl ('purify yourselves') for the state of major impurity (janabah).

Furthermore, he noted that performing ghusl for major impurity also suffices for wudu. One does not need to perform a separate wudu after a valid ghusl, because the lesser purification is encompassed by the greater one.