Walking is great exercise; human beings are designed to walk long distances. Click the link for a great introduction. Try walking 30 min/day; even better if you can do it with a buddy, or your kids.
For some very overweight/obese people, simply walking will get the pulse up high enough for a training effect. That’s why I believe pulse (heart rate) monitoring is useful – not just to make sure you’re working hard enough, but also to make sure you’re not overdoing. If you’ve been a couch potato for years, don’t expect to run a marathon the first day out (or to keep up with the folks who’ve been going to the gym every day for years). Go at your own pace, measure, record, and make slow advances. Your target heart rate range decreases with age.
For those with significant obesity and/or health problems, a visit to a doctor before starting a program may be useful. This website includes an 8 question checklist that may be a guide to whether you should see a doctor before starting an exercise program.