Here are some fundamental and common Linux commands with example usage:
Lists the content of the current directory (or one that is specified). Can be used with the -l flag to display additional information (permissions, owner, group, size, date and timestamp of last edit) about each file and directory in a list format. The -a flag allows you to view files beginning with . (i.e. dotfiles).
Changes the current directory to the one specified. Can use relative (i.e. cd directoryA) or absolute (i.e. cd /home/pi/directoryA) paths.
Displays the name of the current working directory, i.e. pwd will output something like /home/pi.
Makes a new directory, e.g. mkdir newDir would create the directory newDir in the present working directory.
Remove empty directories, e.g. rmdir oldDir will remove the directory oldDir only if it is empty.
Removes the specified file (or recursively from a directory when used with -r). Be careful with this! Files deleted in this way are mostly gone for good!
Makes a copy of a file and places it at the specified location (essentially doing a 'copy-paste'), for example - cp ~/fileA /home/otherUser/ would copy the file fileA from your home directory to that of the user otherUser (assuming you have permission to copy it there!). This command can either take FILE FILE (cp fileA fileB), FILE DIR (cp fileA /directoryB/) or -r DIR DIR (which recursively copies the contents of directories) as arguments.
Moves a file and places it at the specified location (so where cp performs a 'copy-paste', mv performs a 'cut-paste'). The usage is similar to cp, so mv ~/fileA /home/otherUser/ would move the file fileA from your home directory to that of the user otherUser. This command can either take FILE FILE (mv fileA fileB), FILE DIR (mv fileA /directoryB/) or DIR DIR (mv /directoryB /directoryC) as arguments. This command is also useful as a method to rename files and directories after they've been created.
Either sets the last modified time-stamp of the specified file(s) or creates it if it does not already exist.
Lists the contents of file(s), e.g. cat thisFile will display the contents of thisFile. Can be used to list the contents of multiple files, i.e. cat *.txt will list the contents of all .txt files in the current directory.
Displays the beginning of a file. Can be used with -n to specify the number of lines to show (by default 10), or with -c to specify the number of bytes.
Displays the end of a file. The starting point in the file can be specified either through -b for 512 byte blocks, -c for bytes, or -n for number of lines.
Utility usually used to check if communication can be made with another host. Can be used with default settings by just specifying a hostname (e.g. ping raspberrypi.org) or an IP address (e.g. ping 8.8.8.8). Can specify the number of packets to send with the -c flag.
Network exploration and scanning tool. Can return port and OS information about a host or a range of hosts. Running just nmap will display the options available as well as example usage.
Displays the current hostname of the system. A privileged (super) user can set the hostname to a new one by supplying it as an argument (e.g. hostname new-host).
Displays the network configuration details for the interfaces on the current system when run without any arguments (i.e. ifconfig). By supplying the command with the name of an interface (e.g. eth0 or lo) you can then alter the configuration (check the man-page for more details).