The Heavenly Hosts were angel-like service robots that worked for Max Capricorn Starcruisers. They were commanded by Max Capricorn himself, and under his command attacked the Starship Titantic in 2008 for his financial gain. However, after his defeat, they obeyed the next authority figure - the Doctor - and aided him in his saving of the Titanic.
The Hosts speach pattern involves starting a sentence with "information" and then the answer to a query, for example "Information - you are all going to die". Protocol 1 allows the person who executes it to ask three questions, which temporarily overrides whatever command they have until the three questions have been asked.
The Hosts used the Halos on their heads as weapons and flung them at enemies. Hosts like all robots are destroyed in the event of an energy wave as demonstrated when Bannakaffalatta used his cybernetic body to fry alot of the hosts. (DW: Voyage of the Damned)