Repetier and repetier.com are projects concerned with 3D printer software. 'Repetier' is usually use synonymous for repetier-firmware, which is a 3D printer firmware. Use the 'repetier-host' tag if your question is about the host software.
repetier-host is used to send g-code to 3D printers.
repetier-firmware is firmware for the Arduino based control boards of RepRap based 3D Printers.
Repetier is usually synonymous for