曾经一直认为这两个概念是一回事，后来慢慢发现还是有些区别的。

下面是世界卫生组织定义的：Physical activity is defined as any bodily movement produced by skeletal muscles that requires energy expenditure. Physical inactivity has been identified as the fourth leading risk factor for global mortality causing an estimated 3.2 million deaths globally. （from：http://www.who.int/topics/physical_activity/en/）

 

Physical activity is defined as movement that involves contraction of your muscles. Any of the activities we do throughout the day that involve movement — housework, gardening, walking, climbing stairs — are examples of physical activity.

Exercise is a specific form of physical activity — planned, purposeful physical activity performed with the intention of acquiring fitness or other health benefits, says David Bassett, Jr., PhD, a professor in the department of exercise, sport, and leisure studies at the University of Tennessee, Knoxville. Working out at a health club, swimming, cycling, running, and sports, like golf and tennis, are all forms of exercise.

Physical Activity and Exercise: Understanding the Difference

Most daily physical activity is considered light to moderate in intensity. There are certain health benefits that can only be accomplished with more strenuous physical activity, however. Improvement in cardiovascular fitness is one example. Jogging or running provides greater cardiovascular benefit than walking at a leisurely pace, for instance. Additionally, enhanced fitness doesn't just depend of what physical activity you do, it also depends on how vigorously and for how long you continue the activity. That’s why it’s important to exercise within your target heart rate range when doing cardio, for example, to reach a certain level of intensity.

Physical Activity and Exercise: Understanding Intensity

How can you tell if an activity is considered moderate or vigorous in intensity? If you can talk while performing it, it's moderate. If you need to stop to catch your breath after saying just a few words, it's vigorous. Depending on your fitness level, a game of doubles tennis would probably be moderate in intensity, while a singles game would be more vigorous. Likewise, ballroom dancing would be moderate, but aerobic dancing would be considered vigorous. Again, it's not just your choice of activity, it's how much exertion it requires.

Physical Activity and Exercise: Components of Physical Fitness

Ideally, an exercise program should include elements designed to improve each of these components:

• Cardio-respiratory endurance. Enhance your respiratory endurance — your ability to engage in aerobic exercise — through activities such as brisk walking, jogging, running, cycling, swimming, jumping rope, rowing, or cross-country skiing. As you reach distance or intensity goals, reset them higher or switch to a different activity to keep challenging yourself.
• Muscular strength. You can increase muscular strength most effectively by lifting weights, using either free weights like barbells and dumbbells or weight machines.
• Muscular endurance. Improve your endurance through calisthenics (conditioning exercises), weight training, and activities such as running or swimming.
• Flexibility. Work to increase your level of flexibility through stretching exercises that are done as part of your workout or through a discipline like yoga or pilates that incorporates stretching.

While it's possible to address all of these fitness components with a physically active lifestyle, an exercise program can help you achieve even greater benefits.

Increasing the amount of physical activity in your everyday life is a good start — like parking a few blocks from your destination to get in some walking. But to really achieve fitness goals, you’ll want to incorporate structured, vigorous activities into your schedule to help you attain even more of your fitness and health goals.

这段来自：http://www.everydayhealth.com/fitness/basics/difference-between-exercise-and-physical-activity.aspx