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 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.
Introduction to Unix also contains a useful Unix Command Summary page with brief, but useful, explanations of commonly used Unix commands.
Unix is a Four Letter Word... is another resource suitable for newbies.