I think we've covered a lot of information so far, and this lesson will discuss how familiarity works with wand charms and then we'll look at the Unforgivable Curses. I will require at the beginning of this lesson, for all wands to be brought to me so there will be no mishaps, no one will be mistakenly harmed in my classroom *cough* this term.
Now, beginning with familiarity, why do you think, in Harry Potter and the Philosopher's/Sorcerer's Stone that Quirrell didn't just Accio the Stone to himself? He most likely couldn't because magic requires many elements, but a prime one is familiarity. That is why most spells are line-of-sight. The wizard must see what it is in order to charm/curse it. This has been said many times. [An excellent example is when Quirrell and Snape were both casting spells on Harry in the Quidditch match. As Hermione pointed out later, it was imperative that a caster maintain eye contact.]
But then the question arises about how Harry could Accio his Firebolt when he couldn't actually see it. First of all, Harry knows his Firebolt inside and out and he's very 'in tune' with it. He also knows exactly where it is, but I think his familiarity would be enough.
I imagine that there are different levels of power to such magics. All wizards with any training can Accio a visible object. Some, more powerful wizards, would be able to Accio an object that is familiar and if the location is known. But only a very powerful wizard could Accio an object on familiarity alone.
So, Quirrell had no idea what the Stone looked like as there was no picture and it was in Gringott's for ages. He's not totally familiar with it and doesn't know it's actual whereabouts. So, because of those three points, he didn't even try to Accio the Stone, which is a good thing as it would not have worked.
But there are a few possible questions to this theory, specifically with regards to the Accio spell. Mrs. Weasley uses it to take things out of Fred and George's clothing, things she wasn't sure what or where they were. How can this work? She may have seen them put something in their pockets, but that's not likely. Or she's so familiar with the regular items they generally stash in their robes and she sees the robes bulging with objects, which is the more likely idea.
For example, if a wizard saw a tablecloth over an obvious object underneath, that person could Accio it into his or her hand. Of course, this could be verrrrry dangerous. It could be a bowling ball, with teeth or something.
It would be different if there were an object in a tablecloth in a room not in view. Then there's nothing there as far as the person knows, hence, no Accio.
So then what about the Aqualung Harry briefly considered Accio-ing from a nearby Muggle village for the second task? The idea was shot down in short order, so we don't hear it planned out in detail, but it seems likely that Harry would have had to make a trek into that town ahead of time and figure out where such an unusual object might be located and make himself familiar with it. If he didn't it would be impossible to have Accio-ed it. Familiarity is key.
I think this can apply to Portkeys and other spells as well. You can't create a Portkey to somewhere you do not know, you might not even be able to Apparate to a place you do not know. How did the Death Eaters Apparate to Voldemort in the graveyard? Voldemort sent up a "flare" with the Dark Mark tattoo, a sort of Death Eater homing beacon. (1)
Onto the Unforgivable Curses. Let's begin by looking at the Imperius Curse. Impero, which means to govern, order, or command. So this spell will give the caster complete control over their victim's body and mind. I call them a victim because it is a complete violation of your person. Lord Voldemort's Death Eaters used this curse to make innocent people commit many heinous and disgusting crimes. These are people who would never harm a fly, who had to watch themselves, murder other innocent people without thought, merely because they were like puppets with Lord Voldemort pulling the strings. *nods* Unforgivable, yah...
The Cruciatus Curse. Crucio, said KROO-see-oh also affects the mind and body. In Latin, crucio as a verb, means torment. This curse causes excruciating pain and as such, prolonged use can cause the victim to lose their sanity. Some of us know the feeling of a sliver being removed, or a bone being broken. Using this curse goes far beyond any pain that a normal person has ever had to endure. Almost intolerable, it would be a natural response for the brain to shut down functioning normally, due to the overload of stimuli.
Both of these curses will affect the target's mind. There is a rarely taught branch of magic called Occlumency and it is the only defense found against these two curses. It is the art of magically defending the mind against external penetration — the defensive counter to Legilimency. A person who studies Legilimency learns to delve into a person's mind to manipulate them. With Occlumency, you learn to clear your mind of thought and emotion so that the Legilimens trying to delve you cannot see that which you do not want him to see. The word Occlumency stems from the Latin for hide or conceal, occulto, while the ending, mens, means mind.
Lastly, Avada Kadavra. Said uh-VAH-duh kuh-DAH-vrah, we believe it derived from the Aramaic adhadda kedhabhra, meaning 'let the thing be destroyed'. As the first two Unforgivable Curses have the possibility of deflection, through Occlumency for instance, Avada Kadavra has no such counter-curse or means of blocking it. It kills on contact, with a green flash of light at the target and all life ceases. During Lord Voldemort's rise, any unexplained muggle deaths where no cause of death could be found, was attributed to this curse, through his Death Eaters.
(1) Taken from excerpts from Theories in Magic essays from HP Lexicon.