Also known as a "supplemental needs trust", Special Needs Trusts are used to provide support to a minor or adult child with mental of physical disabilities. The idea behind the trust is to allow incrimental support for the beneficiary without jeopardizing their eligibility for means-tested benefits like social security, Medicaid, or food stamp benefits for the beneficiary.
A distribution of as much as $2,000 can make someone ineligible for these benefits ad while $733/month (the maximum monthly SSI benefit) does not seem like a lot of money to miss out on, losing Medicaid benefits as a person with disabilities could be devestating.
Generally speaking, the trust must be discretionary and must be restricted to only allow distributions for the support of the beneficiary, so long as it won't kick the beneficiary off of their means-tested benefits. They are complex planning instruments and must comply with both federal and state law.
It's best to consult a Estate Planning Attorney before attempting to create a Special Needs Trust.