The Holly Robot was a mobile construct that allowed the artificial intelligence Holly to attain a form of solidity and presence, and to move around in remotely. It is called the Holly Robot in the cast and crew DVD commentaries, but was also referred to as the Remote Holly Monitor in the episode scripts.
It was, essentially, a monitor on caterpillar tracks. The frame was apparently steel, with a headlamp fixed to the front. Holly could also download to the Holly Watch.
The Holly Robot was introduced in "Better Than Life" the second episode of Series II, as Holly joins the boys from the Dwarf in "Paradise". The Holly Robot features in every further episode of that series. Holly also joined the crew in this form as they went for a raucous party on a planet to celebrate Rimmer's Deathday. (RD: Thanks for the Memory) The Holly Robot was seen numerous times in the episode "Queeg", as Holly is demoted to nightwatchman duties by Queeg in the Holly Robot form.
From Series III onwards, the Holly Robot is not seen (and the Holly actor changed from Norman Lovett to actress Hattie Hayridge). However the female-headed Holly would often appear in Kryten's chest monitor, since Kryten was now a main character, allowing Holly to appear other places. Such a monitor, for Holly to appear on, was also placed in the space bike used by Rimmer on Waxworld. (RD: Meltdown)