The short answer: He's a sociopathic half-man, half-robot (not really; he just acts like it) who likes smoking cigarettes and, um, shooting people.
But we're not really in the short answer game, at least not when it comes to characters as (in)famous as our Meursault.
The protagonist-narrator of his absurdist adventures, Meursault is a detached and deathly honest guy who refuses to lie about himself to save his life; a simple man, whose moods are painfully dictated by the powers of Nature; and an independent man, one who will not accept God, or any of society’s formulas for happiness.
He's also a jerk.
Can we easily identify with Meursault? Um, well: yes and no. He's young and a bit ambitionless (we're all been there, right?), he is employed by a shipping company in Algiers (okay, sure—we've all had jobs we felt pretty "meh" about), and he isn't interested in exploring opportunities for growth.
But there's a distinct dark side to Meursault even before he pulls the trigger. He's nonchalant and not particularly dutiful—he sends his mother to an old folks' home away in the country without the semblance of guilt. He's kind of a sex fiend and he's emotionally detached—he is fine marrying or not marrying, and he doesn’t see a difference between being in love and being in lust.
But even if such a jaded, uninterested nihilist is hard to identify with, you'd better think again before you label him as a loser.
Because Meursault, for all his many faults, is also simply misunderstood. Far from insensitive, he is attentive to the smallest details. Far from nonchalant, he adamantly refuses to believe in life after death, to seek God out to escape execution, to mask his calmness about or acceptance of death.
But Meursault didn't start out that way in the text. He had to do some pretty serious developing to get that way.
If you look on the surface, it's hard to see any change within Meursault. He starts off uninterested in life, and he ends up…uninterested in life. But there's nothing we at Shmoop hate more than boring ol' surface-skimming analysis. Actually, there are some enormously important, changes going on underneath Meursault's calm exterior.
We're going to break down Meursault's grand evolution into four pieces. Think of this as the Meursault pie.
The first slice of dubiously delicious Mersault pie: Meursault makes no decisions at the beginning of the book. If he's happy, it's because he is passively so. If he's "annoyed," he is passively annoyed. Meursault can't even commit actions on his own. Marriage, no marriage, who cares? As he has told us time and time again, it's all the same, either way.
After all, he doesn't even shoot the Arab, right? According to his narration, "the trigger gave." Now, there is an interesting (and incredibly important) line, before the murder, when Meursault says,
It was then that I realized that you could either shoot or not shoot. (1.6.18)
He comprehends the existence of a choice—but only for a brief moment. After that, he’s back to Mr. Indecision.
Slice #2: notice how dispassionate Meursault is in anything—at least at first. He simply doesn't care. "Annoyed" is the closest he ever gets to being angry, a simple "happy" (which comes across as meaning "content") the nearest he comes to joy. This guy is one cold fish.
Slice #3: check out Meursault's lack of introspection and self-knowledge at the beginning of the novel: he's clueless. He doesn't know himself. The caretaker asks why he doesn't want to see his mother's body, and his response is, "I don't know." He's not aware of his surroundings and, more importantly, he's not aware of his own motivations (and therefore seems to think there aren't any).
Slice #4: look at how Meursault views people at the beginning of his narration. He either wants nothing to do with them (remember how he tries to avoid conversation with the man on the bus?), or he falls casually into supposed "friendships" (like with Raymond).
Either way, he's more interested in the path of least resistance than any sort of connection. But more importantly is the way that Meursault can't understand people. He observes them carefully, he says, "not one detail of their faces or clothes escape" him, but it is still "hard for [him] to believe they really exist." He does follow that strange woman from Celeste's diner, but only because "[he doesn't] have anything to do." He also "[forgets] about her a few minutes later."
So there are your four pieces of the Meursault pie: when things start out, Meursault is passive, dispassionate, ignorant of himself, and unable to connect with or even acknowledge other people. But all of these change throughout the ordeal that Meursault suffers. And that process of suffering is key—Meursault doesn't have a grand "Ah ha! I'm a jerk!" moment.
Although his "revelation" comes at dawn while he's alone in his cell, it is clear that the process was a gradual one and that, in order to get to this epiphany, he had to suffer. A bunch.
So what exactly is this "epiphany?" To begin, Meursault stops being passive; through his actions and words, he makes a choice to yell at the chaplain, then to sleep, then to wake, then to go forward into his death. His actions might not be revolutionary in themselves, but he is aware of them now, conscious, "ready to live it [his life] all again."
When he wakes up, calmer, the next morning, we see that he has moved from a state of indifference to one of acceptance—a fine distinction, but an important one in The Stranger.
And when he wakes up, Meursault is passion personified. When he screams at the chaplain, he does so with both "cries of anger and cries of joy." This guy is awake and roaring—this is something the Meursault of Chapter One could never have done.
He is also certain of everything. In fact, he's "sure about [himself], about everything, surer than [the chaplain] could ever be, sure of [his] life and sure of the death [he has] waiting for [him]."
Most interesting is the switch in the way that Meursault views people. No longer sentencing himself to social isolation, he speaks of "a large crowd of spectators" attending his execution, a crowd that may "greet [him] with cries of hate," such that he feel "less alone."
Where did that come from!? Just a page or two earlier, actually, during his ranting and raving at the chaplain. Meursault says that "we're all elected by the same fate," that with this common fate (death), no one is privileged above any other person—therefore everyone is privileged. (It's interesting that he could have gone in the other direction and said that therefore no one was privileged, but maybe Camus was feeling optimistic.)
If everyone is made common by death, then he can comprehend these other, living creatures as being just like him. Stars: They're Just Like Us! (in that they die.) This counters his earlier statement, when he said it was hard to believe that the residents of the nursing home existed. They must exist—because they are going to die.
Cheery? Nope. True? Sadly, yeah.
Now one last "but": look carefully at the language of this last sentence. Meursault says:
For everything to be consummated, for me to feel less alone; I had only to wish that there be a large crowd of spectators the day of my execution and that they greet me with cries of hate. (2.5.26)
He doesn't actually say he wants to feel less alone. He hasn't yet put into action his revelation. He hasn't yet wished for the large crowd of spectators. He knows he could—he knows this is the next step, but he doesn't do it yet.
One of the major points of The Stranger isn't Meursault suddenly wanting people around—the point is his newfound knowledge, his awareness, and his consciousness.
As long as he knows that he could wish for people (people that hate him at that, which is another can of worms altogether), that's good enough. Stick a fork in this absurdist: he's done.