I never thought I had trouble explaining Twitter to someone. It's a low barrier to entry microblogging system that lets you easily share the therbligs of your life with your friends, family, and followers. No... I don't say it quite that way. I am the kind of guy who would use the word therblig in a sentence, but only if I were there to explain it. Only those of us tortured with the concepts of time and motion studies in college industrial management classes have probably ever seen the word. The point is that Twitter lets you easily share the little stuff in your life. The sphere of those around you can decide what they want to do with it.
Should all of this seem a bit dense, may I point you to the very clear description of Twitter found below:
On Twitter, I am Thinkhammer
Look up: therblig