So we know that Negi's a godboy pimp. But what can he actually do?
Worth noting here that Negima primarily plays it loose and fast. A block of time will have passed, some characters mention that others look like they've gotten stronger, and that's it. No belaboured training stuff here. Consequently, pinning down some of Negi's abilities - particularly which and how many exact spells he can use - can be tough. As an example, during a tournament match, Negi pulls out his staff to do spear techniques on a suspecting Setsuna - Ku Fei merely comments by bringing up there and then she'd been teaching a bit of that, too. If it's awesome, it works.
Though I talk Negi up a lot in this essay, remember that everyone else in the canon gets roughly the same rules. Everyone gets a lot better really damn fast.
The Staff ... is actually just a flying stick, really. Mages use staves and wands to focus their magical energy and cast spells, which is all very good except that halfway through the series Evangeline furnished Negi with a focus ring that could quite happily take it's place. Nowadays, Negi only really needs his staff for flying - which, admittedly, it can do really damn fast when he wants it to - and very occasionally while fighting, as pointed out above.
Other Miscellanous Magical Crap - Negi has a basis in and love of magical gadgets and gizmos, and might learn something about an item by taking a night or two to poke at it. He's also capable of brewing potions up to and including somewhat poor quality love potions, if he's got the gear to do so. Back home he collects magical bazookas.
The Magic Itself - This is extremely rough and ready, but here's an overview: a spell in Negima roughly has two components, the actual spell itself and the element a mage channels into it. This choice of element subverts or adds to the spell's power, shaping it and changing the overall look and effect. Negi excels at wind and lightning and dabbles in light - which, given his propensity for dark magic, appears to be nothing more than a 'default' element. Spells are further made more awesome by how many or how much of an effect you can conjure at one time.
As an example, the typical default attack spell for any mage is 'Sagitta Magica', a straight-up magic missile of light that does damage. If you're a mage worth your salt, though, you're going to throw out ten or more in a single blast, up to over one hundred if you're really awesome or a loli vampire is kicking your ass into training really hard. If you want your enemy to stay down, lob a wind version at them and they'll get bound by magic, or throw a lightning one to try to paralyse them.
Negi's also got a defensive barrier up at almost all times that is pretty much reflexive as long as he's not too battered, and an all-purpose stat-boosting spell that makes him about as naturally strong as a full adult... prior to all his training. He's also got an array of random utility spells such as disarming an opponent or causing a gust of wind, and has one spell he has never used - the power to outright kill a high-level demon forever. It is vaguely suggested in-canon that casting such a spell would rather seriously drain him and requires the demon incapacitated.
Martial Arts - Negi is a stupendously quick learner and tends to end up in situations that give him a lot of very urgent practice. Combined with his boosting himself via magic he's a skilled combatant, and has already become good enough at Chinese Kenpo that he can at least be a bit of a match for his master. (Ku Fei - not the loli, that's the other master. The two mom effect, only one is abusive.) The styles he typically uses make full use of his tininess and slipperiness, and benefit greatly from his flash-stepping around in battle to launch surprise attacks; elbows, shoulderbarges and trips feature prominently.
Spoiler - Yeah, uh, you probably won't see this in camp, at least until canon update. More on this as required.
None of these are his strongest powers, though.
Adaptability and Just Being Gar: Negi gets thoughts and ideas and, in the heat of battle, figures out ways to use his existing abilities in new and helpful methods. By taking the kenpo he's learnt and weaving some western style and a lot of magic into it, he's coming up with his own fighting style where - in effect - what he's doing is punching or charging someone with multiple magic missiles around his fist.
It's also been said many times that Negi isn't actually one of the strongest people in his canon, and that certainly prior to the Magic Kingdom arc most of the real fighters in his class and Kotarou could all smoke him with relative ease. Negi excels at the pinpoint final chances, the random and wild last-ditch plans, and the all-out desperation.
Finally, he just. Keeps. The Fuck. Going. Like the mini-Kamina he is, he will stand up against an opponent again and again, way after he should have given up and limped off home already. And if pushed far enough into this state, he's liable to pull off his equivalent of a tantrum; an angry magic-fuelled rage worthy of the Thousand Master's son.