LINUX IS CASE-SENSITIVE. For example: Mozilla, MOZILLA, mOzilla, and mozilla would be four different commands (but of the four, only mozilla is available on my system). Also my_filE,my_file, and my_FILE are three different files. Your user login name and password are also case sensitive. (This goes with the tradition of UNIX and the "c" programming language being case sensitive.)
Filenames can be up to 256 characters long and can contain letters, numbers, "." (dots), "_" (underscores), "-" (dashes), plus some other non-recommended characters.
Files with names starting with "." are normally not shown by the ls (list) or dir command. Think of these "dot" files as "hidden". Use ls -a (list with the option "all") to see these files.
"/" is an equivalent to DOS "\" (root directory, meaning the parent of all other directories, or a separator between a directory name and a subdirectory or filename). For example, try cd /usr/doc.
Under Linux, all directories appear under a single directory tree (there are no DOS-style drive letters). This means directories and files from all physical devices (plus network filesystems) are merged into this single-view tree.
In a configuration file, a line starting with # is a comment. When changing a configuration file, don't delete old settings--comment out the original lines with #. Always insert a short comment describing what you have done (for your own benefit!).
Linux is inherently multi-user. Your personal settings (and all other personal files) are in your home directory which is /home/your_user_login_name. Many settings are kept in files with names starting with a dot "."so as to keep them out of your way (see point 3 above). You have to login to do anything.
System-wide settings are kept in the directory /etc .
Under Linux, as in any multiuser operating system, directories and files have an owner and a set of permissions. You will typically be allowed to write only to your home directory which is /home/your_user_login_name. Learn to use the file permissions else you will be constantly annoyed with Linux.
Command options are introduced by a dash, "-", followed by a single letter (or -- when the option is more than one letter). Thus "-" is an equivalent of DOS's switch "/". For example, try rm –help.
Type command& (the command name followed by an &) to start a command in the background. This is usually the preferred way of starting a program from the X-window terminal.