A backtick is a single quote mark slanted from upper left to lower right. You might also call it a accent aigu or acute accent. When used as quoting characters, these are referred as backticks.
Backticks are one of the more useful shell tricks. Simply put, the shell replaces back ticks with the output of whatever was between the back ticks. An example:
$ less `locate fstab`
This locates all files containing fstab somewhere in their paths and displays them with less.