Sentinels

The Sentinels were a race of robots built by the Janitor to help him clean up the ridiculous amount of dirt that had built up on the Halo Rings. They were armed with lasers that fired burning Cillit Bang at dirt and mess, designed to lift out stains and clean even the toughest of grime buildups.

Unluckily for the Galaxy, the idiots at the Forerunner Council thought that the Sentinels were a battle robot and replaced the Prometheans, highly advanced albiet annoying fighting machines, with these new cleaning droids. Their weapons proved inneffective against the Flood, no surprises there.

The only thing that the Sentinels did of any use during the Human-Covenant war was take shots for their allies. For example, during the battle of the Ark, Guilty Guilty summoned hundreds of Sentinels to go to Truth's Citadel, so that the Grunts would waste all their ammo destroying them.

Different Types of Sentinel
There are various different forms of Forerunner Artificial 'Intelligence' that patrol the Halo Installations, most others were hastily built by the Janitor to make up for the fact that his cleaning robots were being used as warriors. Knowing that they wouldn't be very effective, the Janitor started adding new forms of Sentinel to enhance their overall combat abilities:

Sentinel Major
These are basically normal Sentinels, but they are gold. They are fitted with energy shields and fire a blue beam of Cillit Bang, which isn't just set on fire, its superheated to massively hot temperatures to burn through Flood flesh. Some of them also come armed with Needlers, for some reason. Despite the fact that they are seemingly just as ineffective against the Flood as normal Sentinels, the Majors were able to keep the 100,000 years war with the Flood on Halo Installation 05 at a stalemate for centuries, so they must be doing something right (for once...)

Sentinel Enforcer
Also known as simply 'Enforcers', these are huge Sentinels with large Energy Shields that cover the front half of their body. Originally used for vermin control, these have been refitted for use against large Flood infestations and have various forms of attack. They either use their pulse beams to bombard targets with red spikes that explode on impact, or they can use their mortar launchers to assault enemy vehicles with explosive projectiles. If all else fails, they can lift up enemies and crush them with their huge arms, which were originally used in Forerunner day care for dealing with screaming children.

Constructor
These are tiny, plasma pistol-shaped robots that fly around fixing any damage to Halo Installations with their tiny green beams. No-one knows how the beams actually repair things, or if they even can repair things, but they can be used as a weak offensive weapon if the Constructor is in danger. Weak but agile, the Constructors were originally used to clean toilets by the Janitor, and so were few in number (despite the extraordinarily high amount of shit that Forerunners seem to produce).