Marriages in Islam are social contracts and have to be understood in two separate ways: the mechanics of the actual marriage contract and the responsibilities/rights associated with the marriage.
There are many cultural practices that are common however there are only a few that are religiously required (as per traditional interpretations- I won’t be dealing with other interpretations in this post).
One of the first requisites by Islam is the right to chose ones own spouse. Neither sides can/should be forced into a reunion against their will.
Like all contracts, marriages are written and signed by the bride and groom with two witnesses from each side (2 from the groom, 2 from the bride), done in the presence of an imam or a justice of the court. That means that there are a minimum of 7 people involved in any marriage contract (though they might not all be present simultaneously).
The contract itself sets the terms of the “mahr”- the amount that the husband owes the wife after the consummation of the marriage. This is the opposite of dowry. In divorce proceedings there are various rules associated with the return of the mahr. Additionally, any other clause of legally permissible items can be added.
One more concept that is of note is that there are separate contracts for separate “transactions” in Islam. This is no different for marriages where a marriage cannot be conditional on another marriage. The terms of the contract (mahr, clauses etc) are embedded in itself, not on any other marriage contract.
The rights and the responsibilities of husband and wife have to be understood as counterparts. The right of the wife is the responsibility of the husband and vice versa. These can of course be manipulated/customized based on individuals.
The main one is associated with finances. Men are required to cover all living expenses for their wives and families. The traditional religious counterpart for women is the responsibility of taking care of the household.
There are many other aspects of marriages however these are a few of the main ones. So why the primer on marriage is Islam? Because many of the various types of marriages mentioned in my previous post violate one or more of these requirements. Some are invalid, while others are considered reprehensible (foul, heinous, shameful).