If your area has been converted to digital, you'll need a cable box or DTA on every analog TV. Digital TVs may not need a box, but might need a CableCard to decode Extended Basic channels. You need a cable box on any TV's that you want to be able to view channels in the Digital tiers on (including premium channels like HBO and Showtime).
Depends. If you have a digital tv, you can connect to the cable directly and receive local channels (usually channels 2-30) without a box. Higher channels will require a box to receive them. If you live in an area with good over the air reception, you can also use an antenna to receive local channels.
Comcast is moving channels from analog to digital so higher channels will not be accessible with a box. You can enter in your account number here to see what you'll need to continue receiving the channels in your package. There are FAQs here about this.