OctoPrint is a so called host software for 3D printers that controls your 3D printer and sends it the actual commands to do its job. Other tools for this task include Printrun, Repetier Host and also Cura.
OctoPrint differs from existing host solutions in that its major focus is to provide a web interface that allows controlling the printer remotely from anywhere on the network or even the internet while offering the same responsiveness and feedback options available on native host applications like the ones listed above. In order to achieve this, OctoPrint makes heavy use of current web technologies and frameworks, such as AJAX and HTML5 web sockets.
OctoPrint was developed to be run on small embedded devices such as the popular Raspberry Pi. It allows you to turn your existing 3D printer into a WiFi enabled one and untether it from your laptop or work station.