A will is a legal instrument by which a person, called a testator, disposes of his property. The will is written before his death and takes effect after his death. Usually an executor is named to carry into effect the testator's wishes and witnesses sign the will.
There are other types of wills that you may run into in your research. A nuncupative will is an oral will spoken during the testator's last illness before witnesses and reduced to writing and signed by the witnesses. This type of will is often called a death bed will. Some states accept them as legal and other states do not. One of the benefits of a nuncupative will is that the death date is often given.
A holographic will is entirely in the handwriting of the testator with no witnesses. This type of will is often written during an emergency. For example, a hiker is lost in the woods, knowing he is close to death, scribbles his wishes regarding his estate on a piece of paper. Again, states vary of which accept this type of will.
There are other wills, but the above three are the ones you will encounter most often.
Reference: Black's Law Dictionary, Sixth Edition, 1990.
Published 29 March 2016, Western Kentucky Genealogy Blog, http://wkygenealogy.blogspot.com/