In Simek's words, hamingja is "the personification of the good fortune of a person. It is understood not only abstractly but also as a kind of soul-like protective spirit, and thus is closely associated with the fyljur." The idea of mobile spirits who "hovered beside or near a person in life" were the common property of Nordic peoples, according to Thomas DuBois. The later Christian idea of guardian angels is similar but guardian angels are not transferable, nor are saints, and are bound to one person at a time. The idea of various protective spirits, fylgjur and hamingja, lends to confusion in our understanding of how these spirits were conceived. The use of the word hamingja is comparable with that of fylgja. It is compounded with hamr (skin, shape), whichi s occasionally used to mean '"fetch".
Luck plays a very important role in life, most especially in times of conflict, and a man known to be lucky found it easy to gather followers - and to fend off opposition. When King Harold was marching across Norway, subduing one region after another, King Audbjorn of the Firthfolk tried to recruit Kvedulf to his cause, but the doughty old leader declined:

'It is my duty to the king to take the field with him if he have to defend his own land, and there be harrying against the Firthfolk; but this I deem clean beyond my duty, to go north to Mæra and defend their land. Briefly ye may say when ye meet your king that Kveldulf will sit at home during this rush to war, nor will he gather forces nor leave his home to fight with Harold Shockhead. For I think that he has a whole load of good-fortune where our king has not a handful.' (Egilssaga 3)

In the event, Kvedulf was proven right, and Harold triumphed over those who opposed him in Audbjorn's name. Hamingja may be given out by those that hold it and have perfect control of it. It is generally passed down to one person in the kin-line.
It should be stressed that hamingja does not necessarily have anything to do with one's own actions in life. Grettir possessed bravery in abundance, as we see by his defeat of the berserks who menaced Thorolf's farm (Grettir's Saga 19 but of hamingja, nothing (78). It should be noted that Davidson (Road to Hel, 132) misinterprets this episode as saying that the old woman simply states this situation as a fact to Grettir, when in fact, she puts a curse on him:

The woman was lying in the stern sheets covered up with clothes. Then she began to stir and said:
"These men are brave and unfortunate; there is much difference between you; you offer them good and they refuse everything. There are few more certain tokens of evil than not to know how to accept the good. Now I say this of you, Grettir, that you be deprived of health, of all good luck and fortune, of all protection and counsel, ever the more the longer you live. I wish that your days may be less happy in the future than they have been in the past."

Parents would often name their children after a departed ancestor in the hopes of attracting that individual's hamingja to the child. We see examples of this in Vatnsdæla Saga 7: ...The boy shall be called Ingimundr after his mother’s father, and I hope for luck (hamingja) for him on account of the name and in Finnboga Saga 36 where a dying man begs his son to name a child after him because he said he was sure that hamingja would follow’.
We are shown how this transference is made in Viga Glúm's Saga 9:

It is told that one night Glúmr had a dream. He thought he was standing outside the house, and looking towards the firth. He thought he saw a woman walking across the country, and coming towards Þverá. She was so huge that her shoulders touched the mountains on each side. He thought he went out of the homestead to meet her, and asked her to his house. And after that he awoke. All thought this dream strange, but he said ‘This is a great and remarkable dream, and I would read it thus: Vigfúss my grandfather must be dead, and the woman who was higher than the mountains as she walked must be his hamingja, for he was nearly always above other men in honour; his hamingja now must be seeking an abode where I am’ .

Although H.R. Davidson wants to attach this episode to that experienced by Hallfreðar (Hallfreðar Saga 11), there is clearly no connection outside of the appearance of the women. One brought ill-luck to Hallfreða, the other, honor and distinction to Glúmr, as would be expected of hamingja. (Davidson, Road to Hel, 131)
A man, particularly a king, may also lend his hamingja to another while he yet lives. Turville-Petre goes so far as to call it "kingly force." Hamingja is generally used in abstract senses of ínborn force, luck'. But when a man 'lends' his hamingja to another, as kings often do, the sense is more nearly concrete" (Turville-Petre, 230). An example of this is how Christian King Ólaf Tryggvason lent his luck to his attendant Thorstein.

Thomas A. DuBois, Nordic Religions in the Viking Age (University of Pennsylvania Press, 1999), 52.
H.R. Ellis-Davidson, The Road to Hel (New York: Greenwood Press, 1968), 131.
Rudolf Simek, Dictionary of Northern Mythology (Cambridge: D.S. Brewer, 1993), 129.
E.O.G. Turville-Petre, Myth and Religion of the North, 31, 230