One unique feature in ZSH is that you can redirect streams to multiple outputs or inputs simultaneously. With this multi-stream redirection you can, e.g., redirect STDOUT OR STDERR of a program to more than one file at the same time without using a command line tool such as tee.
File descriptors are very essential in programming. When you open a file, you get a token, which identifies the file instance. With this token you refer to the file, write, read or close it. You have about the same on the shell.
All Linux administrators know this tool and some make use of it very often: cat. Also Linux beginners get to know this quickly and use it a lot. Most people love it so much that they utilize it in every situation whether it's needed or absolute nonsense.
One little tool which's often overlooked is tee. It is very smart and can help you so much. tee is a typical UNIX program. It can't do much and has been created for only one purpose: writing STDIN to STDOUT and a file. That doesn't sound very exiting yet but this simple tool can do a lot for you.