twitter concepts

hello. i was very glad to see the app on tweetlist, which i do use, but my use of twitter is very limited. all i do is follow various sites, such as applevis, southeastern guide dogs, serotek and others. i cannot wrap my head around the whole twitter thing. i don't get the concept of tweeting, retweeting, and some of the other functions discussed in the app. is there a really good book that anyone can recommend for getting the whole twitter thing down so it does not fly past me. thanks. maybe i am getting too old for this stuff!!!