Forensic Graphology is the study of handwriting especially that found in ransom notes, poison pen letters or blackmail demands.
Handwriting, Typewriting, Shoeprints, and Tire Treads : FBI Laboratory's Questioned Documents Unit (QDU) personnel at the FBI Laboratory support federal, state, local, and international law enforcement agencies by providing advanced technical and forensic support. Physical examinations in the QDU involve close and careful scrutiny of surfaces and other items. Paper is the most typical surface examined.
How handwriting analysis works
Forensic handwriting analysis is about comparing two or more documents and determining within a reasonable degree of certainty that the same person or that different people wrote them.
Handwriting Examination: Meeting the Challenges of Science and the Law FBI : Forensic Science Communications.
Forensic of handwriting analysis
A handwriting analyst with forensic training can determine, by comparing a document with samples of the same handwriting, if it is real or a forgery.
Questioned document examination
QDE is the forensic science discipline pertaining to documents that are (or may be) in dispute in a court of law. The primary purpose of questioned/forensic document examination is to answer questions about a disputed document using a variety of scientific processes and methods. Wikipedia article - compare information against a verified source.