Remark, observation, comment, commentary, note, obiter dictum can all denote a brief expression intended to enlighten, clarify, or express an opinion.
A remark is a more or less casual expression in speech or writing of an opinion or judgment (as of something seen in passing, something read for the first time, or something to which one’s attention has been called); the term usually carries no implication of a final or considered judgment.
Observation may suggest a reasoned judgment based on more or less careful scrutiny of the evidence.
Comment applies to a remark or an observation made in criticism, in interpretation, or in elucidation of something.
Commentary may be used in place of comment for an annotation or gloss of a passage or text. More often, however, it is employed as a collective noun, designating a series of annotations or glosses provided for the elucidation of a text or literary work.
Even more often the word is used of a running sequence of oral comments (as on a sports event or the news) made on radio or television or sometimes accompanying a film projection.
Note applies chiefly to a written or printed comment or gloss on a particular point (as the historical origin of an idea, the exact meaning of a term, or the source of the writer’s information) made either by the reader of an article or book on the margin of a page or by the author or editor to be printed at the bottom of the page (then called footnote ) or, with other comments or glosses, in an appendix.
Note is also applicable to brief statement jotted down (as one of the minutes of meeting, a memorandum of a point developed or to be developed in a speech, or of a point made by a speaker or lecturer).
Obiter dictum is applied in law to an incidental opinion delivered by a judge on a matter bearing upon but not material to the case being tried, and therefore having no binding force. In general use it is usually applied to a remark or observation made more or less on the spur of the moment and not intended to be taken as a final opinion or definitive statement.