When someone puts something into trust they are the Settlor and give up all legal ownership of the assets they put into trust. Legal ownership passes to one or more trustees who are now the legal owners. Usually there is one or more beneficaries of the trust, these are the people that can benefit from the assets held in trust.
Of course trust deed documents detail the rules of the trust, how and when the trust assets can be used or disposed of, how and when the trustees can be changed and how the beneficaries can be changed.
For example a discretionary trust may be set up such that the trustees get paid x amount of pounds a year for managing the trust, and that the beneficaries can be removed as beneficaries if they get a legal conviction and that the beneficaries only get benefit if the trustee decides to do so.
Let’s say a trustee likes his annual fees and doesn’t want to give the beneficaries anything as he will thereby maximise his fees from the trust funds. Further that the trust holds properties that if disposed of by the trust (e.g. gifted to a beneficiary) would incur a massive capital gains tax bill for the trust which the trustee might not want, so they instead decide to sell the properties thereby generating more cash-flow for their fees. This is based on a real-life example I know of.
Discretionary trusts can be very dangerous and are usually used where there is no real-life trust of the beneficaries.
However, not all trusts are equal, for example all non-government DB pensions are held in trusts of one form or another.