Also known as an "A" trust, the marital trust provides management for assets passing to your spouse. The alternative is to leave assets for your spouse outright (no trust). A trust can add a level of comfort that someone is available to manage the assets. If no restrictions are placed on what happens to the assets after the second spouse dies, it's called a general power appointment. If you choose to control what happens after the second death, you need to establish a Qualified Terminable Interest Property trust or "QTIP" trust (a stricter form of marital trust).
sometimes referred to as "Trust A" in an "A-B Plan," it is a trust that is structured to hold assets for the benefit of a surviving spouse, and still qualify for the marital deduction. A Marital Trust pays all of its income to the surviving spouse. Additionally, the spouse may be entitled to certain amounts of principal upon request.