As the name implies, primary progressive aphasia is a disorder of language (aphasia) that is progressive in nature and not due to neurological diseases that are easily seen on an MRI like stroke, head trauma, and cancer.  There are three distinct forms of primary progressive aphasia-- semantic variant Primary Progressive Aphasia, nonfluent/agrammatic variant of Primary Progressive Aphasia and the logopenic variant of Primary Progressive Aphasia.  These are important to recognize because they are important clinical screening tools for the underlying pathology causing each disorder.