To puff means to breathe or blow heavily. When followed by the preposition on, it is often a synonym for smoke.
Ex: Jon was puffing and panting from running, so he decided to take a break. After a few minutes, he had recovered enough to continue.
Ex: The old man puffed on his pipe as he rocked in the rocking chair on his porch and watched the sunset.
Another way to say that someone is breathing heavily is huff and puff. Huff is a noun meaning a state of annoyance or anger, such as in the expression in a huff.
Ex: In the classic story about the three little pigs, the big bad wolf huffed and puffed to gather enough breath to be able to blow the pigs' houses down.
Ex: Yasmin left the house in a huff because she was annoyed that her flatmate had used some of her kitchenware.
The word casual refers to something that happens by chance, not on purpose. It implies that there is no serious thought or concern.
Ex: Was Julie offended by what I said about her haircut? It was a casual observation, so I hope there is no misunderstanding.
Ex: I ran into my boss at a restaurant on Saturday night. It was a casual meeting, so neither one of us mentioned work.
The word casual can also refer to an informal style of clothing. This includes jeans, tshirts, and any clothing that could not be worn to a formal occasion.
Ex: I am going to wear something casual to the party. We will be on the pool deck, and I do not want to stumble in a pair of dress shoes.
Ex: Because of the warm summer weather, we are allowed to wear casual clothes to the office. The men would be much too hot in heavy suits.