The basic difference between a Pour-Over Will and a testamentary trust is that the testamentary trust is created by instructions in the trustor's Will and only comes into effect after the trustor passes away. A Pour-Over Will is created while the trustor is alive and transfers any missed property directly into an existing Living Trust without creating an entirely new trust.
For example, non-testamentary trusts, like a Revocable Living Trust, take effect as soon as the trustor signs the document and has it notarized. In contrast, testamentary trusts are often used when a trustor wants to leave assets to a beneficiary, like a minor child, but only wants the beneficiary to receive the property once they reach a certain age.