Probate records refer to wills, inventories, letters of administration and guardianships. Not all of our ancestors left wills and if you search only for wills you are likely to overlook valuable records. There are three basic types of wills. In Colonial days this was often done by local magistrates, and in recent times by lawyers. All three types must be signed by witnesses.
Attested wills are the most common ones.
Witnesses could not be beneficiaries, but were often relatives. So, pay attention to the names of witnesses. Some confusing terms in wills may lead you to incorrect assumptions. In-law may have been used in the modern sense, but could just as easily refer to a stepchild. The term cousin probably causes the most problems.