In today's high-tech world, a legal signature goes beyond scrawling your John Hancock in the proper place on a document. Whether a signature must be handwritten in ink or if other forms are valid depends on state law, the instructions on the document or contract, and the nature of the transaction. In many situations, pre-stamped, electronic or facsimile signatures are legal. Your intent when signing a legal document -- not your penmanship -- is what counts.
In 2000, Congress passed the Electronic Signatures in Global and National Commerce Act, often called the E-sign Act. It permits the use of an electronic "signature" to satisfy legal signature requirements, as long as the individual consents to this use and his consent remains current.
Documents such as wills generally require two individuals who aren't beneficiaries to witness the signature. Other types of documents, including various financial transactions, require a notary public to witness a signature. An illiterate person requires witnesses when he writes his "X" on a document, possibly along with proof that he understands the purpose of the document and any agreement or consent given by his "X."