What is the difference between aspirin and ibuprofen?

What is the difference between aspirin and ibuprofen?

http://goaskalice.columbia.edu/aspirin-vs-ibuprofen

http://www.diffen.com/difference/Aspirin_vs_Ibuprofen

http://www.health.harvard.edu/fhg/updates/12-things-you-should-know-about-pain-relievers.shtml

There are many other links, too.  These links include other over-the-counter (OTC) pain medications, too.