In 11th century England, “hearth companion” was a general term for a lord’s household troops or personal bodyguards. However, hearth companions often did more than simply bear arms, and they were also known by other names: house-carls, huscarls, or gesitha. The terms changed and took on different meanings throughout the Anglo-Saxon and Viking period–there was even a time in which hearth companion and thegn meant the same thing.
https://talesofmercia.wordpress.com/2010/08/03/history-of-he...

The term ‘huscarl’ meant ‘hearth companion’ and this reflects one of the chief aspects of the post. Originally it was the king who would be attended by such men as he was rich enough to pay them. The Viking kings of the Dane-Law in England are believed to have introduced this idea and it was copied by the Saxons, especially those lords rich enough.

The Saxon huscarl was not just a companion but also a bodyguard to their lord. They swore a death-oath to protect him. If their lord fell in battle then the huscarls were honour bound to either kill all of his enemies or die on the same field. If they should survive the battle then they would recover their lord’s body and see it given a fitting funeral.
http://www.petercwhitaker.co.uk/page15.html

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Note added at 10 hrs (2015-11-16 20:16:37 GMT)
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The primary purpose of the hearth companions was of a military nature. In battle, they directly surrounded and fought with their lord. But when they were not at war, the duty of the hearth companion became more malleable. “In times of peace, the king’s hearth companions were the nearest equivalent to a police force: they administered his laws and enforced his royal authority”.