In the Qur'an, Allāh ta'ala uses three words to describe our hearts: qalb, fu'aad and sadr. We know that every Arabic word is chosen for a reason, yet all three of these words for the most part are translated as heart. As usual, the intricate meanings of these words are not captured in the translation.
So, what is their difference?
Qalb is the general word for heart. It comes from the root which means something that turns around and about and upside down. It is the nature of hearts that they are constantly changing, this is the normal state of our hearts. When Allāh ta'ala refers to emaan and diseases of the heart, qalb is used.
Fu'aad is from the verb fa'ada meaning burning or a flame and lahmun fa'eed means a roasted meat on a fire. Fu'aad is used when the heart is inflamed with emotion, as they say, in the heat of the moment. Fu'aad is used in the Qur'an to describe the heart when it is engrossed in emotion: happiness, sadness, lust, frustration, anger or regret. My favorite example of this is in suratul Qasas (28), Allāh ta'ala says,
But there came to be a void in the heart of the mother of Musa. (28:10)
Allāh ta'ala uses fu'aad to describe the heart of Umm Musa after putting her newborn into the Nile, as if it was about to leave her chest in her worry and sadness. Allāh ta'ala says herfu'aad became faarigha, empty. She was in such an emotional state that she could not think of anything except Musa alayhi salaam.
Then He says, 'She was almost going to disclose his (case), had We not strengthened her heart.' (28:10)
After Allāh ta'ala strengthened her heart out of His Mercy, when she regained her emaan, patience and trust in Him, her fu'aad became qalb.
Another example of fu'aad is when Allāh ta'ala says:
Indeed, the hearing, the sight and the heart all those will be questioned. (17:36)
Allāh azza wa jal did not say our quloob will be questioned, rather He ta'ala said the fu'aad.
When people are committing sins, some type of emotion overtakes them which makes them cross the limits. How many times have we heard, It's not me, it's the hormones! When we are questioned on the Day of Judgment, we cannot use the they were a fitnah, I couldn't help it!, I was emotional! or it was out of anger! excuse on that Day to Allāh ta'ala because we are the ones in control of our fu'aad. It's not going to work. We will be accountable for what our hands, eyes and hearing reaped, no one else.
Sadr takes the meaning of chest, and when Allāh ta'ala eludes to motives or secrets, he uses sadr because it gives the illusion literally of a treasure chest: something hidden and boxed up.
This example can be seen in suratun Naas:
The one who whispers in the hearts of Mankind. (114: 5)
Shaytan does not make waswas (whispers) into our quloob, but rather he whispers in the chests of the people. There is a distinction because Allāh ta'ala created the qalb as pure. The example of the heart is like a fort and the eyes/tongues/ears/hands are gates to the opening. Shaytan waits outside of this fort, attacking the openings trying to get in. Out of Allāh's Rahmah, He did not give Shaytan access to our quloob, but rather to our sudoor. It is us who let the gates open, not Shaytan.
These seemingly simple words contain an ocean of wisdom behind the use of each. We all understand that the Qur'an is a book of wisdom, but when we study intricate details like this, we truly see HOW it is a book of wisdom. We ask Allāh ta'ala to make us amongst its companions, āmīn.
Allāh is Most Knowing.