Shmoop.com is another helpful education site. It has overviews of poetry, literature, and U.S. History topics plus a book club. Each topic is handled in neat, bite-size portions that allow you to go as in-depth as you need into your topic. I like that each topic has a brief overview and a "why should I care" section that is very helpful for those questions on why something is significant. For history, there is a timeline for each event and sections on how that event affected different minority groups and economics groups. Literature has sections for summary, quotes, and themes. Each topic has a study question guide which is great for making sure you really understand the material and are ready for that upcoming test.
Website; 14+ (high school +); Shmoop.com