Fee Simple Estates: the who gets what of interpreting the last will and testament of your deceased British or Irish relative.
Fee Simple estates are freehold estates - a remnant of feudal times and have an infinite ownership period. The owner is entitled to the land forever, or at least until they dispose of it.
Disposition can be done by sale, in which case the infinite right to ownership is simply transferred by conveyance to the buyer. Complications arise when the estate is a gift, especially if it appears on the pages of a will.
The gift may be absolute, in which case the will simply states "To A in fee simple". It only gets complicated when the testator decides to introduce Words of Limitation; these are words which will reduce the ownership period of the estate to something less than infinity. This can be done by saying "To A, for life", this creates a life estate which obviously differs from a fee simple unless the new owner happens to be immortal, or "to A and his heirs"; this creates a fee tail estate - in which the land is passed down through the generations.
Determinable Estates: this is where your dead relative seeks to control your life from beyond the grave by inserting clauses such as "I leave my thousand acre estate to A so long as he becomes a doctor, otherwise it shall be given to Rufus, my budgerigar". This is known as a determinable estate. If A becomes a doctor, he is granted ownership, if he does not, the land goes to the budgie, who is known legally as the remainderman.
There can be conditions precedent (such as the one above), where the beneficiary must complete a task to inherit the land, and conditions subsequent, in which case you get the land, just as long as you don't marry a member of a rival family, stop being a doctor, etc. A good example of a condition subsequent is the one set in Brewster's Millions.
There are of course rules regarding the nature of the conditions set. Note that if a condition is deemed ridiculous or invalid by the courts, then you simply inherit all the land as a fee simple estate. Conditions which are impossible to perform are invalid (Re Elliott) and those which are just plain nasty are also invalid (Re Waring's Will Trusts, where the testator bequeathed all to his nieces on the proviso that they never married).