In the common law, a will or testament is a document by which a person (the testator) regulates the rights of others over his property or family after death. In the strictest sense, "will" is a general term, while "testament" applies only to dispositions of personal property (this distinction is seldom observed). A will is also used as the instrument in a trust.

Any person over the age of 18 can draft his own will without the aid of an attorney. Additional requirements may vary, depending on the jurisdiction, but every will must contain the following:

The testator must clearly identify himself as the maker of the will, and that a will is being made; this is commonly called "publication" of the will, and is typically satisfied by the words "last will and testament" on the face of the document.

The testator must declare that he revokes all previously-made wills and codicils. Otherwise, a subsequently-made will revokes earlier wills and codicils only to the extent that they are inconsistent. However, if a subsequent will is completely inconsistent with an earlier one, that earlier will be considered completely revoked by implication.

The testator must demonstrate that he has the capacity to dispose of his property, and does so freely and willingly.

The testator must sign and date the will, usually in the presence of at least two disinterested witnesses (persons who are not beneficiaries).

The testator's signature must be placed at the end of the will. If this is not observed, any text following the signature will be ignored, or the entire will may be invalidated if what comes after the signature is so material that ignoring it would defeat the testator's intentions.

After the testator has died, a probate proceeding may be initiated in court to determine the validity of the will, i.e., whether it satisfied the legal requirements, and to appoint an executor. If the will is ruled invalid in probate, then inheritance will occur under the laws of intestacy as if a will were never drafted.

Although there is no legal requirement that a will be drawn up by a lawyer, there are many pitfalls into which home-made wills may fall, and it is highly desirable that any will is the subject of legal advice before drafting or execution. The person who makes a will is not available to explain him or herself, or to correct any technical deficiency or error in expression, when it comes into effect on that person's death, and so there is no room for mistake.

A very common error (for example) in the execution of home-made wills is to use a beneficiary (typically a spouse or other close family members) as a witness, although this has the effect in law of disinheriting the witness regardless of the provisions of the will.

Some states recognize a holographic will, made out entirely in the testator's own hand. A minority of states even recognize the validity of nuncupative (oral) wills.

A will may not include a requirement that an heir commit an illegal, immoral, or other act against public policy as a condition of receipt. In community property jurisdictions, a will cannot be used to disinherit a surviving spouse, who is entitled to at least a portion of the testator's estate.

It is not only a good idea, but also essential that the testator give his executor the power to pay debts, taxes, and administration expenses (probate, etc.).

Revocation is the process of disavowing a previously executed will.

The intentional physical destruction of a will by the testator will revoke it. This could be accomplished by the testator deliberately burning or tearing the physical document itself, or even by striking out the signature. Most jurisdictions allow partial revocation if only part of the text or a particular provision is crossed out. Other jurisdictions will either ignore the attempt or hold that the entire will was actually revoked. A testator may also be able to revoke by the physical act of another (as would be necessary if he is physically incapacitated), if this is done in his presence and in the presence of witnesses. Some jurisdictions may presume that a will has been destroyed if it was last seen in the possession of the testator but is found mutilated or cannot be found after his death.

A will may also be revoked by the execution of a new will. Most wills contain stock language that expressly revokes any wills that came before them, however, because normally a court will still attempt to read the wills together to the extent they are consistent.

In some jurisdictions, the complete revocation of a will automatically revives the next most recent will, while others hold that revocation leaves the testator with no will so that his heirs will instead inherit by intestate succession.

Where a will has been accidentally destroyed, on evidence that this is the case, a copy will or draft will may be admitted to probate.

Many jurisdictions exercise an equitable doctrine known as dependent relative revocation. Under this doctrine, courts may disregard a revocation that was based on a mistake of law on the part of the testator as to the effect of the revocation. For example, if a testator mistakenly believes that an earlier will can be revived by the revocation of a later will, the court will ignore the later revocation if the later will comes closer to fulfilling the testator's intent than not having a will at all. The Doctrine of dependent Relative Revocation also applies when a testator executes a second (or new) will and revokes his old will under the (mistaken) belief that the new will would be valid. However, for some reason the new will is not valid and the courts may apply this Doctrine to reinstate and probate the old will, as they believe the Testator would prefer the old will to Intestate Succession. 

http://en.wikipedia.org/wiki/Will_%28law%29