Comic - A medium of storytelling using visual interpretation but not necessarily combined with text, although used as a general term for this medium, it is often used to refer to a book of, but not specifically 32 pages. Comic series are usually published on a monthly basis by teams of people.
Comic Book - As above a comic book is a book, or more like a magazine, staple bound and paperback of 32 pages. It could also refer to a comic of longer length than usual or a graphic novel, however I feel perhaps the best use of this term is for single issues of a comic series or a book that collects several comic issues of a series.
Graphic Novel - The equivalent of a novel for comics, usually a full story or a volume of a series. These are created by one person or two, a writer and an artist, as with comics. However it's more common for graphic novels to be created by one person in comparison to comics.
Webcomic - Comics that are uploaded and displayed on websites. Usually self driven projects in which people show their work for free.
Visual Narrative - Another word for the medium of storytelling which images are employed to convey a message.
Sequential Art - As previous, both terms are used as an alternative to comics/graphic novel although they can be used for other areas of art.