It's a testament to how far Linux has come that users today don't typically have to use the command line if they don't want to. Such is the quality of the graphical user interfaces in many modern Linux distributions that there's simply no need, in general.
Yet the command line can be a highly efficient way of getting things done in the Linux world--it's often a much simpler, easier and more direct method than clicking through all the menu choices, in fact.
I believe fear of the command line is one of the top mistakes newcomers to Linux sometimes make. For that reason, let's look at some of the main commands any Linux user should know.
I begin with the "man" command because in many ways it's the most fundamental. This command is what you should use when you want to learn more about another command.
Essentially, "man" displays online manual pages to provide a basic introduction to the various other commands that are available. Using it can help you understand the functions of those commands as well as how to use them. You can even type "man man" to get the man page for "man" itself.
If you're ever unsure of a command you're about to use, try typing "man" along with the command to learn more about exactly what it does.
Another fundamental command is "ls," which displays the contents of your current directory. For example, "ls ~" will show you the files in your home directory.
To find out which directory you're working in, type "pwd," which is short for "print working directory." Typed in the desktop directory, for example, this command will return "~/desktop."
To change the directory you're in, use this command. To switch to the desktop directory, for instance, you'd type "cd desktop."
To create a new directory, this is the command you'd use. For example, "mkdir pictures" will create a directory called "pictures."
6. find and locate
To search for a file, you can use either "find" or "locate," both of which are very similar.
This is the command to use when you want to create a copy of a file with a new name. Type "cp cats dogs," for instance, and you'll get an exact copy of "cats" named "dogs"; the file "cats" will still be there too.
With "mv," you can change the name of a file or move it to another directory--or both. Typing "mv cats dogs" will rename the file "cats" to "dogs," for example, while typing "mv cats ~/desktop" will move the file "cats" to the desktop directory without renaming it.
The "more" command allows you to see the contents of a file one screen at a time.
When you want to stop an application, there's no more efficient way to do it. Typically "kill" is used after "ps," which lets you find out the process ID of what you want to kill.
Part of the reason Linux is so secure is that only the root user has the privileges needed to install or remove applications and make other big changes. To install an application as a regular user, the sudo command can give you those root privileges temporarily. You will, however, have to enter your password.
Need to change your password? Then just type this command at the prompt, and it will ask you to type the current password and then a new one.
Keep in mind that the documentation that goes with the Linux distribution you're using can probably help. In Ubuntu, for example, the help section on the command line is a nice place to learn about the various commands and what you can do with them.
Linux.org's "Getting Started with Linux" tutorial is also a great place to explore.