Fee simple, also known as fee simple absolute, is an estate in land in common law. It is the most common way real estate is owned in common law countries. Allodial Title is the most complete ownership interest that can be had in real property. How ownership is limited by government powers such as taxation, eminent domain, police power, and escheat often involves the shift from allodial title to fee simple such as when uniting with other property owners ceding to property restrictions or municipal regulation.
In English common law theory, the Crown has radical title or the allodium of all land in England, meaning that it is the ultimate "owner" of all land. However the Crown can grant an abstract entity -- called an estate in land, which is what is owned. The fee simple estate is also called "estate in fee simple" or "fee-simple title." and sometimes simply freehold in England and Wales. In the early Norman period, the holder of an estate in fee simple could not sell it, but instead could grant subordinate fee simple estates to third parties in the same parcel of land, a process known as "subinfeudation." The Statute of Quia Emptores adopted in 1290 abolished subinfeudation and instead allowed the sale of fee simple estates. 
Historically estates could be limited in time, such as a life estate, which is an interest in lands that terminates upon the grantee's (or another person's) death, even if the land had been granted to a third party, or a term of years . It also could be limited in the way that it was inherited, such as by what was called an entailment which created a fee tail. Traditionally fee tail was created by words of grant such as "to N. and the male heirs of his body"; which would restrict those who could inherit the property. When all those heirs ran out the property would revert back to the original grantor's heirs. Most common law countries have abolished entailment by statute; but life estates can still be created (although they are uncommon).
Other estates in land include the defeasible estates. Defeasible means that the estate can be annulled or voided. Examples of a defeasible estate are the fee simple determinable (aka determinable fee) and the fee simple on condition subsequent.
The concept of a "fee" has its origins in feudalism. According to William Blackstone, the great common law commentator, fee simple is the estate in land which a person has when the lands are given to him and his heirs absolutely, without any end or limit put to his estate. Land held in fee simple can be conveyed to whomever its owner pleases; it can be mortgaged or put up as security as well. No rent or similar obligations are due from the owner of property in fee simple.
Etymology: Fee - A right in law to the use of land; i.e. a fief. Simple - in the unconstrained sense: (1) without limit to the inheritance of heirs; (2) unrestricted as to transfer of ownership.