A statutory form is a form created by a government, usually designed to serve as a model form or a free form for the public. The text of the form resides within the government's statutes.
For example, many states have statutory durable powers of attorney forms written into their laws. In these states, you should ideally be able to go into the statutes, copy the form, and complete it. As an example, see the Ohio statutory power of attorney at Ohio Revised Code § 1337.60. States may also provide more user-friendly versions of the form outside of the statutes (often on the state's website) -- for example, here is the Illinois statutory power of attorney for health care.
The easiest way to find a statutory form in your state is to do an internet search for the name of your state and the name of the form. For example, you could search for "California statutory will." If you get too many results from non-state sources, try limiting your search to just ".gov" sites.
The advantages of using statutory forms include getting it for free and the certainty of knowing that the form itself is meets legal requirements.
However, the disadvantages of using statutory forms are numerous and more grave. Because legislatures design statutory forms for the masses, they are one-size-fits-all and rarely flexible enough to meet individualized needs. In contrast, a lawyer can usually craft a document that takes your unique situation into account. Also, statutory forms may not come with instructions that explain how to correctly fill out and finalize the form, so you will probably need to educate yourself about how the form works, how to sign it, and other legal or practical issues.
While governments provide statutory forms for convenience and clarity, they are rarely mandatory. That is, in most cases, states also allow you to use non-statutory forms. So even though your state may provide a statutory will or power of attorney, a form drafted by an attorney or produced with form-making software will also be acceptable -- provided that it adheres to the requirements for non-statutory versions of the form.
If you have any trouble, questions, or concerns about using a statutory form, see a lawyer for help.