A bog-standard example of a trust is set up where property is held on trust for someone, ie. I hold my house on trust for my kids. This involves a splitting of ownership of property into legal ownership and beneficial ownership; ie. I am the legal owner of the house, but my kids own it beneficially, that is to say, in equity.
Trusts can also exist even without being formally set up, for example when it's unfair to adhere to the law strictly. So, if Bob has a beneficial ownership in my house, and I sell the house to Charlie who knows that Bob has beneficial ownership, Charlie will hold the house on trust for himself and Bob.
Gets a lot more complex, but there are some interesting areas, such as Charitable Trusts, and Imperfect Gifts.
Trusts actually is the largest syllabus out of any subject, so much that the exam paper is split into Part A and Part B to ensure that teachers know what to teach out of the 15 or so parts.