We have tried to put all the basics / overview of Hadoop in one place. We have named it Hadoop Basics.
Going through the online course might help you to understand the basic aspects of Hadoop.
You could start from this course.
The course is open to all, no sign up required, free of cost.
On the right side of the online course, an index would be visible where all the topics are listed.
By clicking, on the topic link, you would be able to reach directly to the topic.