Called "the greatest contribution of the anglo-saxon to jurisprudence" (I forget who said that, but he was French), the concept of the trust, and of equitable title form an integral part of the theory of property in English jurisprudence.
In English law, there are two kinds of title: Equitable title, and legal title. Legal title is the right to bring common law actions concerning the ownership of the property (essentially); equitable title is the right to enjoy that property. Where both kinds of title are not vested in a single person on his own, there is a trust.
An example of this is where I give my friend Dave my shoes for the use of my unborn nephew Bob, but only on Wednesdays. The equitable title vests entirely in Bob, so he may demand that Dave give the legal title over to him also. If I were to have stipulated the term that Charlotte be allowed the shoes on Fridays, then if Dave should give either of them the shoes on a different day than that appointed, the other could sue Dave for breach of trust. They could, of course, demand that he give the shoes over to their joint ownership entirely, again ending the trust.
The consequence of the equitable title theory is that trusts are not, in the ordinary way of things, revocable. Once the equitable title has vested in a beneficiary, the settlor may not revoke the trust. On the other hand, if a trust is created (entirely) for beneficiaries who do not exist yet, or are unascertainable, the equitable title remains with the settlor, and may demand the onjects of the trust back. As the beneficiaries don't actually exist yet, they can't petition the courts to stop this, and the property is (in equity) that of the settlor still.
The origin of the trust
The trust originated during the crusades, when land could only be held by men. Men could claim any old land if there was no other man with a better claim around. This was bad, as your wife and kids could be turfed out onto the street, while you went off to kill infidels. So, many a man entrusted his entire estate to his mate, Lucius. Unfortunately, sometimes Lucius turned out to be less of a friend than thought, and would grab the land for himself, turfing the lady and kids out. On his return, the knight would be like "Holy Fuck! That bastard stole all my shit!". Unfortunately, at common law there was nothing to be done, so the knight had to petition the king to do the equitable thing. Chancery court developed, time passed...
On a side note, the trust was used as an inheritance tax-dodge, by placing all of one's estate with a group of trustees. Before the rule against perpetuity, as long as there were always trustees, no-one inherited, so no tax. Henry VIII didn't like this.
Trusts come in many different forms, and can be classified in many different ways. I do not propose to give the subject a full treatment here (Try Snell's Principles of Equity). I will happily list related nodes below.