Comics are a medium used to express ideas with images, often combined with text or other visual information. It typically takes the form of a sequence of panels of images. Textual devices such as speech balloons, captions, and onomatopoeia can indicate dialogue, narration, sound effects, or other information. There is no consensus among theorists and historians on a definition of comics; some emphasize the combination of images and text, some sequentiality or other image relations, and others historical aspects such as mass reproduction or the use of recurring characters. Cartooning and other forms of illustration are the most common image-making means in comics; Photo comics is a form that uses photographic images. Common forms include comic strips, editorial and gag cartoons, and comic books. Since the late 20th century, bound volumes such as graphic novels, comic albums, and tankōbon have become increasingly common, along with webcomics as well as scientific/medical comics.

Speech balloons are a graphic convention used most commonly in comic books, comics, and cartoons to allow words to be understood as representing a character's speech or thoughts. A formal distinction is often made between the balloon that indicates speech and the one that indicates thoughts; the balloon that conveys thoughts is often referred to as a thought bubble or conversation cloud.