Thought bubbles come in two forms: the chain thought bubble and the "fuzzy" bubble.

The chain thought bubble is the almost universal symbol for thinking in cartoons. It consists of a large, cloud-like bubble containing the text of the thought, with a chain of increasingly smaller circular bubbles leading to the character. Some artists use an elliptical bubble instead of a cloud-shaped one.

Often animal characters like Snoopy and Garfield "talk" using thought bubbles. Thought bubbles may also be used in circumstances when a character is gagged or otherwise unable to speak.

Writers and artists can refuse to use thought bubbles, expressing the action through spoken dialogue and drawing; they are sometimes seen as an inefficient method of expressing thought because they are attached directly to the head of the thinker, unlike methods such as caption boxes, which can be used both as an expression of thought and narration while existing in an entirely different panel from the character thinking. However, they are restricted to the current viewpoint character. An example is Alan Moore and David Lloyd's V for Vendetta, wherein during one chapter, a monologue expressed in captions serves not only to express the thoughts of a character but also the mood, status and actions of three others.

Source: Speech balloon from Wikipedia

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