How is laxative useful for constipation?

I am 26 years old and for the past 4 days I have been suffering from constipation. I do admit here that I am a smoker and my constipation could be due to my smoking habit. Now my mother went to the chemist yesterday and brought one laxative for me. I have not used any such thing so I would appreciate if someone can tell me as to how is a laxative useful for constipation? Tell me its mode of action?