I don't get it = I don't understand. So you can say it after someone tells you a joke or explains something to you. I don't know is what you say if someone asks you for a piece of information such as the time of the next bus or the capital of Mongolia. :)

" I don't get it" is a casual way of saying that you don't understand. If you don't understand a difficult subject such as math you could say "I don't get math" Be careful though, if someone says "I don't get cold" or "I don't get paid" they are referring to a lack of something not that they don't understand. The difference is often in the context of how it is used. "I don't know" means that you don't understand. This can be used in a similar way to "I don't get it" by adding words to the end such as "I don't know him" or "I don't know math". It will always mean you don't understand. If someone asks you a question you don't know the answer to you should use "I don't know" if someone says something you don't understand you can say "I don't get it" OR "I don't know what you said"

"I don't get it" is an informal way of saying "I don't understand" or "I am puzzled." EXAMPLES: I don't get this math problem. = I can't figure out the answer. I don't get it. Why is she mad at me? = Men are from Mars, and women are from Venus. I don't get it. Why is everyone laughing? = I don't understand the punchline of the joke. (This happens to me when the joke is in another language like Spanish. Someone has to explain why it's so funny because I only understand the literal meaning of what was said.)

" I don't get it" is a casual way of saying that you don't understand. If you don't understand a difficult subject such as math you could say "I don't get math" Be careful though, if someone says "I don't get cold" or "I don't get paid" they are referring to a lack of something not that they don't understand. The difference is often in the context of how it is used. "I don't know" means that you don't understand. This can be used in a similar way to "I don't get it" by adding words to the end such as "I don't know him" or "I don't know math". It will always mean you don't understand. If someone asks you a question you don't know the answer to you should use "I don't know" if someone says something you don't understand you can say "I don't get it" OR "I don't know what you said"