The USA DTV changeover date is February 17, 2009.
The Candian DTV changeover date is sometime in 2011.
The USA will be switching away from "analogue" transmissions of "Over The Air" Television signals. The reason for this, is that they can save a lot of wireless spectrum (ie: the amount of space available in the sky to transmit stuff wirelessly) by transmitting it digitally. So, you get the same quality signal (perhaps even better) but they can also run things like more data over new cellphone airwaves.