United States Probate Wills
A will is a written and legal expression of the individual's instructions regarding his property a the time of his death. The will usually describes the estate and often gives the names and relationships of heirs or beneficiaries. The affidavit of the witnesses includes the date or proof of death. Types of wills include written, nuncupative, and holographic.
If accepted by the court, a copy of the will was recorded in a will book or register kept by the clerk of the court. The clerk may have made errors when he transcribed the will, but the original will is often kept in the probate packet.
Probate laws vary by state, but generally a will is valid if the testator was of legal age, of a sound mind, and was free from restraint when he or she wrote the will.
A nuncupative will is an oral will which is later spoken of in court by those who heard the testator speak. Some U.S. states do not honor this type of will. (A to Zax by Barbara Jean Evans, [Alexandria, VA: Hearthside Press, 1995], 186.)
A holographic will is the original copy written by the testator.
Ammendments to wills are called codicils.
The United States Probate portal page explains how to use wills in family history research.