Depends on what type of emergency you want to be prepared for - earthquake, floods, tornadoes, hurricanes, etc. In general, at the very minimum, you should have enough bottled water per person for at least 3 days, plus food (think granola bars, dried foods like for camping, etc.), an emergency blanket for each person, flashlight w/fresh batteries, portable radio w/fresh batteries, a basic first aid kit and some cash (credit card, too, but cash is better to have). If you use prescription meds regularly, you should have a supply of those, too. If you have pets, they need the same type of basic supplies of food, water & meds. If you are in a disaster prone area, copies of important documents (insurance cards, SS card for each family member, etc.) in a sealed bag in your kit is a good idea. You can also buy ready-made kits.
For more information, FEMA has a website: http://www.fema.gov/areyouready/
at 4:02 PM on Apr. 5, 2010