The wikipedia article provides something of an answer, quoting a letter as a source:
http://en.wikipedia.org/wiki/Wizard_(Middle-earth)
They were "clothed" in the bodies of old Men, as the Valar wished them to guide the inhabitants of Middle-earth by persuasion and encouragement, not by force or fear. However, they aged very slowly and were in fact immortal. Physically they were "real" Men, and felt all the urges, pleasures and fears of flesh and blood. Therefore, in spite of their specific and unambiguous goal, the Wizards were capable of human feelings; <PERSON>, for example, felt great affection for the Hobbits. They could also feel negative human emotions such as greed, jealousy, and lust for power. 712
It is hinted in the essay in Unfinished Tales that the Blue Wizards may have fallen prey to these temptations, though information published in The Peoples of Middle-earth seems to contradict this version of their history. Although immortal, their physical bodies could be destroyed by violence—thus <PERSON> truly died in the fight with the <PERSON>, beyond the power of the Valar to resurrect him; <PERSON> himself intervened to send <PERSON> back.[1]
Similarly, <PERSON> was killed when his throat was cut by his own servant in The Return of the King. The Istari also carried staves, which seem to be tied to their ability to wield magic; when <PERSON> is defeated at Isengard, <PERSON> in the same breath casts him from the White Council and breaks his staff (although he retains the persuasive power of his voice).
And the citation in question:
In Letters, #156, pp 202–3, <PERSON> clearly implies that the 'Authority' who sent <PERSON> back was above the Valar (who are bound by Arda's space and time, while <PERSON> went beyond time). <PERSON> clearly intends this as an example of <PERSON> intervening to change the course of the world.
So they did not, in fact, have access to all powers of the Maiar. Maiar are truly immortal and cannot be killed (as I understand it).
And I may have been wrong about their staves being purely symbolic.
The Istari also carried staves, which seem to be tied to their ability to wield magic; when <PERSON> is defeated at Isengard, <PERSON> in the same breath casts him from the White Council and breaks his staff (although he retains the persuasive power of his voice).
The true answer seems to be: "It is never explained." 381