What constitutes exercise?

I wonder what does constitute exercise.  Does walking constitute exercise?  What type of walking and when?  Do I really even exercise?  Am I still a beginner?  If I am not really exercising, then what I am afraid of?  Is walking in the backyard exercise?  How about walking around the house?  What would typify a slow walk; would it depend on speed or distance?  These are questions that I have to ask and decide for myself if it is exercise.  I am just wondering.