It is derived from ar-ru'yah (seeing/looking/observing). It's foundation is doing actions of goodness and piety so that people see them or hear of them, in order to win their praise and admiration, to gain position and status amongst them and to obtain the goals and objectives of this world.
It is haraam due to the clear texts of the Book and the Sunnah and the chapter regarding the order for ikhlaas and the warning against riyaa' and shirk has already preceded.
1. Riyaa' of the body.
This occurs by appearing to be emaciated and pale so that others may see the intensity of his striving and the abundant fear of the Hereafter. Also included in this is lowering the voice and sinking of the eyes, showing that one has a weak and feeble body in order to show others that he is diligent in observing fast.
2. Riyaa' of the outward appearance.
Such as the effects of prostration on the face or wearing a specific type of clothing which only a few people wear and whom people consider to be scholars. So he wears this apparel so that it may be said: "He is a scholar".
3. Riyaa' by one's speech.
This occurs mostly by admonishing others, reminding them, memorising details and narrations for the sake of argumentation, making it open and know that one has copious and abundant knowledge, moving one's lips for the remembrance of Allaah in the presence of others, becoming angry when evil occurs between people, lowering the voice and making it soft and tender when reciting the Qur'aan so as to show otehrs his fear, sadness and other such things.
4. Riyaa' with one's actions.
Like the showing off of the worshipper by lengthening the standing and prolonging the bowing and prostration during prayer. Outwardly displaying humility and showing off with fasting, going on military expeditions, pilgrimage, giving charity and other such things.
5. Riyaa' on account of one's friends and visitors.
Like the one who burdens himself in order to visit a scholar or a pious worshipper so that it may be said: "So and so visited so and so". This also occurs by inviting people to one's house so that it may be said: "The people of the deen frequently visit him".
Should the Muslim Abandon an Action Due to Fear of Riyaa?
Leaving an action out of fear of falling into riyaa' is one of the biggest mistakes and ways of falling into the trap of Shaytaan. If the intent and motive for an action is correct and righteous and it is in agreement with what has been legislated then the action should never be abandoned due to the presence of the danger of riyaa'. Rather it is obligatory upon the Muslim to strive against this threat and danger. This is achieved by making his heart cling to modesty in front of Allaah and fear of Him, changing the praise and admiration of creation for His praise and admiration.
Fudayl ibn Ayyaad (ra) said, "Doing an action for the sake of people is shirk and leaving an action for the sake of people is riyaa'."
Ikhlaas is when Allaah saves you from both