"To fool someone" means to mislead someone. For example, someone looks really nice but has an awful personality: "Don't be fooled by his appearance, he's a bad guy."

"Be fooled" indicates a passive action, meaning that the object is the recipient of the action of the verb "fooled". Here, fooled means "deceived" or "tricked". So the sentence breaks down quite easily: "Do not" : Negative command. "be fooled" : be the recipient of deception "by..." : indicator of who or what is performing the deception. Compare with Harregarre's example: "Don't be fooled by his appearance, he's a bad guy." "His appearance" is performing the act of deception, and "be fooled" indicates that you are the subject of the action, because of the passive tense.

Harregarre's spot on. To BE FOOLED by something is to MAKE A MISTAKE ABOUT something. The meaning is a little different because in the first case, you are the victim of a trick. Don't be fooled by what she tells you. She is a politician.

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