As a standard rule, a verbal contract is legally binding if all essentials are met within the overall structure and context and you can prove the agreement was actually consummated and therefore exists.
The optimum way to ensure your rights are protected, what was bargained for is performed to completion, and the mitigation of "Gray Areas of Contention" subsequent to agreement inception, is to sign an explicit, WRITTEN, expressed contract with any and all conditions contained therein, including consideration ( What was bargained for etc), and all other applicable terms.
Even written contracts can be challenged, however, precise verbiage contained therein can clarify questionable aspects whereas verbal agreements can be deemed ambiguous or convoluted at best, especially by third party interpretation such as a judge or jury.
As previously mentioned, another potential circumstance which may render the verbal agreement invalid, voidable, non-binding etc, is non-compliance with the "Statue of Frauds" which dictates certain contracts that are required to be reduced to writing before deemed legally binding by a court of law - As the previous example states - The purchase of Real Property - The sale and process of ownership transfer must be performed via precisely crafted and drafted documents containing appropriate signatures, dates, etc. Or, a contract containing terms which must be performed over an extended period of time may also require a written instrument to be considered binding -