One of the requirements of marriage under Islamic law is that the man gives the woman a dower, a gift of money. The dower is usually called mahr although other terms are used as well, particularly sadaq or saduq.
The mahr is the woman's separate and exclusive property and cannot be touched by her husband at any time during the marriage without her express consent. It remains her property in the event of divorce or death of the husband.
The woman's right to the mahr is created when she and the man sign a marriage contract and is perfected after the marriage is consummated. If the couple divorce before consummation and the mahr has not been given, it does not need to be given (Quran 2:236). However, if the couple divorce before consummation and the mahr has been given, the woman is entitled to half (Quran 2:237). Once the marriage is consummated, the mahr becomes the woman's exclusive property (Quran 4:4).
To the best of my knowledge, although it is not a requirement of Islamic law, the custom has developed in many Muslim cultures that the mahr is divided into two parts, the prompt and the postponed. The prompt mahr is given at the time that the marriage contract is signed. It is usually a token amount (e.g., $1). The postponed mahr will be given later. The custom is that the couple agree that the postponed mahr will become due if the couple divorce. Very often, the postponed mahr is a substantial amount (e.g., $50,000). The idea is that the man will be reluctant to divorce when he has to pay all this money. Since the husband is obligated to provide complete financial support for his wife during the marriage, she has no need of the mahr during that time. Instead, it serves as security for her after the divorce.
It should be kept in mind, however, that the mahr can only be postponed to divorce by the consent and agreement of the woman. If she chooses to demand the full mahr after the marriage is consummated, she may do so. It is her right.
The Mahr provides a brief overview of what Islamic law says about the mahr.