The word "will" has numerous meanings, but in personal finances it usually refers to a legal declaration of a person’s (or testator) desires regarding the disposal of their property and estate after death. In modern times, a will is almost always a document. After death, a will goes through a legal process colloquially known as probate. Part of this process is to ascertain the validity and legality of the will. For this and other reasons, wills should be developed with the assistance of qualified legal advice (particularly if the will involves a trust).
Wills can be revoked or changed such that the "last" will is deemed authoritative. You may often hear the phrase "last will and testament"-"will" is a generic term while "testament" applies specifically to the disposition of personal property, though in modern use the distinction is usually deemed irrelevant.