No, OctoPrint and OctoPi are not the same thing.
OctoPrint is a fully featured print server/web interface for 3D printers, written in Python and HTML/JS. It is platform agnostic and runs basically everywhere where you can install Python 2.7, be it a cheap single PCB computer like the Raspberry Pi running Linux, a fully featured gaming PC running Windows, a Mac running MacOSX or a server running some BSD flavor. It was created and is actively maintained by @foosel.
OctoPi on the other hand is a preconfigured image for the Raspberry Pi. It is based on the Raspbian Lite image by the Raspberry Pi Foundation and bundles OctoPrint plus mjpg-streamer as a webcam server plus CuraEngine plus a couple of scripts to make your life easier and allow for easy initial setup. This image is probably the most common form of running OctoPrint. It was created and is actively maintained by Guy Sheffer.
Think of it this way: OctoPi is more or less the operating system, OctoPrint is the main application that runs on it and actually talks to your printer.