Unity's launcher is a user interface element designed to both provide quick access to frequently used applications, as well as display and group currently running applications.
The launcher sits at the left side of the screen. The dash icon is at the top, followed by icons for applications. The workspaces icon typically follows all applications, and the trash icon is found at the very bottom.
The launcher's size and hiding behavior can be customized through Compizconfig Settings Manager. The location on the left is currently fixed.
Applications can be accessed by either clicking on their icons or by using Super + Number. Hold down Super to see which numbers correspond to which applications.
Applications in the launcher also have quick lists accessed by right clicking on the icons. These can be customized and provide access to common tasks.