An acronym for Uniform Type Identifier - a text string that uniquely identifies a given class or type of item.
Uniform Type Identifiers - or UTI are a replacement for the classic OSType file creator and type system used by older versions of Mac OS to classify types of files. They were introduced with Mac OS X and became the default file type on version 10.4. UTI are more generic than just a file system classifier and also can be used to identify clipboard contents, streaming data, application bundles as well as files. Additionally, the system comes from Apple with many common UTI defined (and reserved) and allows third parties to extend this system with their own types.
These text strings use a reverse DNS naming convention, support most unicode characters, and supports multiple inheritance. UTI are instrumental in finder and launch-services as well as spotlight and quicklook.