Mahr, Explained
Mahr is a mandatory gift from the groom to the bride, hers alone to keep, use, or invest as she chooses. The Qur'an (Surah An-Nisa, 4:4) instructs that women be given their bridal gifts graciously. It's her right, not a transaction with her family.
Who mahr actually belongs to
Mahr is the bride's personal property. It isn't shared with her family, isn't a payment for the marriage itself, and isn't something she's expected to hand over to her husband afterward unless she genuinely chooses to. This is one of the more commonly misunderstood points about mahr, particularly where it gets confused with unrelated cultural practices.
How mahr differs from dowry
Dowry customs, where a bride's family gives money or goods to the groom's family, exist in some cultures but are not an Islamic requirement, and run in the opposite direction to mahr. Conflating the two has caused real confusion and, in some communities, real financial pressure on families that has nothing to do with what Islam actually asks for. Mahr is owed by the groom to the bride. Nothing is owed the other way.
How much, and in what form
There's no fixed amount. It should be something both sides agree is fair and the groom can genuinely afford. Scholars generally discourage setting it so high that it becomes a burden rather than a gift. It doesn't need to be money: a well-known account describes a man with nothing else of value being told he could give his new wife something as simple as an iron ring, or teach her some of what he'd memorised of the Qur'an. Mahr can also be split between an amount given immediately and an amount agreed as owed later.
Mahr belongs to the bride alone, isn't the same as dowry, has no fixed amount, and doesn't need to be money. What matters is that it's genuine and agreed by both sides. It's one part of the wider contract. See the role of the wali for the other core element, or rights and responsibilities of spouses for what follows after the contract itself.