A UGMA account is an account an adult holds and manages for a named child under a state's enactment of the Uniform Gifts to Minors Act. It is the predecessor of the arrangement most people now mean when they say custodial account, and the two are separated by a generation of drafting rather than by any difference in purpose: an adult transfers property to a child, an adult manages it until the child is old enough to receive it, and no trust document is written.
Explaining the naming is most of the work this page does, because the two names are almost identical and only one of them is likely to govern a reader's account. Both are model acts, written for state legislatures to adopt rather than enacted by Congress, so the operative text in any case is the enacting state's own statute. The gifts act came first. The Uniform Transfers to Minors Act came later and was written to replace it, and the replacement is the one a bank or brokerage will open today. The UTMA account page carries how a live custodianship works, including irrevocability, the custodian's duties, the handover age, and the deposit insurance treatment. This page covers what the earlier act was and what becomes of an account opened under it.
The one thing worth fixing in a reader's mind before anything else: an account titled under the gifts act is not void, dormant, or in some legal limbo. It holds real property belonging to a real child, and the money is reachable. What changed is the statute the custodian is answering to.