As Linux is a Unix-like operating system, a general familiarity with fundamental Unix concepts such as the Unix directory hierarchy and file permissions is essential. Fortunately, there are many Unix tutorials online that collectively cover everything a user might need to know to use Linux. I suggest you bookmark the following as an online reference library:
The Wikipedia Unix article is a good place to start. It includes an Unix history, explains some Unix concepts and includes an explanation of basic commands and some relevant links.
Eric S. Raymond's The Unix and Internet Fundamentals HOWTO is a good document to learn about the Unix concepts.
The UNIX Tutorial for Beginners of the University of Surrey is a good step by step tutorial for beginners.
The Command and Shell User's guide from Digital Equipment Corporation is an excellent guide to the Unix Operating System. Bookmark the Index Page linked there as an online reference.
UNIXhelp is mirrored on many servers around the world. It is a useful resource for newbies.
Unix is a Four Letter Word... is another resource suitable for newbies.