Unmarried people (children...usually) receive these red envelopes, known as 红包 (lai see, cantonese) from the married, i.e. parents, aunts and uncles and the like. It's considered good luck to give these, and the red is to ward off evil spirits. I've also heard that it's supposed to be something like a wish for future prosperity.

My house is multilingual. The...children (most of whom are over the age of 14) primarily speak English, while the adults speak Cantonese, Vietnamese, English as well as at least one other Chinese dialect (my mom speaks Hakka chinese). Since my family runs a Vietnamese restaurant, we usually just go with the Vietnamese new years greeting: Chúc mừng năm mới!

Anywho, excuse me while I get some sleep.