A Trust是常说的Marital trust，即婚姻信托。

B Trust是常说的Bypass Trust，即继承信托，也称为避税信托。

A marital trust is a specific type of trust established for the benefit of a surviving spouse. It is a trust that takes advantage of the unlimited marital deduction in order to avoid estate taxes at the time of the first spouse’s death in the event that the first spouse’s individual estate is more than the individual exemption amount.

Here’s how it works: At the time of death, trust-owned assets are transferred to a trust for the benefit of the surviving spouse, essentially allowing estate taxes to be delayed until the second spouse’s death. The surviving spouse must be the only beneficiary of the trust during his/her lifetime, however, at the time of the second spouse’s death, the trust can pass to any other named beneficiaries like children, grandchildren, etc.

Additionally, an A Trust can give the surviving spouse broad access to the funds in the trust during the survivor’s lifetime, even allowing the spouse to withdraw everything in the trust. Finally, A Trusts are irrevocable trusts, which means they cannot be changed or altered after the trust has been established.

B Trusts (also called family trusts or credit shelter trusts) work a little differently but are often used in concert with A Trusts–meaning oftentimes, when a spouse sets up an A Trust, they also set up a B Trust. B Trusts are once again created upon the death of the first spouse, but they’re capped at whatever the current estate tax exemption allows. This means they can greatly reduce your estate tax liability, or, if the entirety of the estate is less than the combined exemptions of both spouses, then a B Trust can help avoid estate taxes altogether.

二者的区别为：

With a marital trust, the surviving spouse generally is able to access the income, as well as the principal balance. However, the principal in a bypass trust can be used for expenses of the surviving spouse, such as health and support, but is not generally accessible to the surviving spouse. Instead, those assets will not be included in the surviving spouse's estate for federal estate tax purposes at his or her death and are passed to the contingent beneficiaries of the bypass trust free of federal estate tax.