"Wow, Yuuno!" you might say. "I've seen a little bit of Magical Girl Lyrical Nanoha and people are shooting giant lasers and swinging huge laser swords around everywhere! You've got to have some sort of totally cool super lasers too, right?"
Well, uh.
Um.
Yuuno really doesn't. See, the thing is, when the first season of MGLN was produced, Yuuno was slated to be a main character with equal billing as far as we know, given his starring role in the first season. His abilities might have grown and evolved along with the others.
Unfortunately, a different director took over for the second season, and this director had far more interest in the... well, Magical Girl. As such, Yuuno got downgraded to a blink-and-you'll-miss-him entry in the OP, and a relegation to second string. StrikerS dialled his appearences down to a mere two in a 26-episode series, so much so that he needed a nametag in subtitles in case people had forgotten who he was.
HOWEVER! This just means that we can play fast and loose with his somewhat less defined abilities :D, so everything's good. Here are the abilities Yuuno has displayed in canon, and the skinny on what they do.
DEFENSIVE ABILITIES
Barrier Jacket- Yuuno's combat suit, which honestly isn't up to much, this is the 'henshin' of the series or what the heroes transform into when they do fighting. As a representation of how little Yuuno has dedicated himself to actual fighting, this is very simple and not much of an upgrade at all to his normal uniform. Most of the time, Yuuno wears this tunic which is cream and green, with shorts (tan), a dark brown t-shirt, a cape and armored boots and gloves. Of this, the only things that actually count as his barrier jacket are the boots and gloves. These can actually be detonated in an emergency using a command code.
Round Shield- This is one of Yuuno's personal defense abilities. He creates one of the series' trademark images- a circular magic shield with geometric patterns and a ring of runes- which he can use to shield himself from attack. This might not sound like much, but consider this- in the season opener to A's, Yuuno faced off against Vita, one of the Wolkenritter. Vita's designated as the member of the group focused on pure destruction, and there's supposedly nothing she can't destroy. Using this shield, Yuuno held her off perfectly for an entire fight. Now that's some defensive power.
Round Guarder Extend- This is one of Yuuno's non-personal abilities. It's comparable to a 'buff' in that what it creates is a stationary shield and magic circle around another person, which can then be used independantly of him. It's supposedly very useful for protecting weakened mages. He used it on Nanoha a lot.
Circle Protection- This one's rather flashy. It's another shield, but this one creates a very large barrier which will discharge special effects and lightning when you come into contact with it, but the problem is that it's somewhat easier to penetrate. It's not as strong as Nanoha's, which might mean that it's an ability linked to offensive magic- it counterattacks when someone hits it.
Sphere Protection- As above, but in a spherical configuration. Simpler than some of Hayate's rubbish!
OTHER STUFF
Jewel Seed Seal- One of Yuuno's few offensive abilities, although this is what wiki refers to it as it's probably just because it was only used on a Jewel Seed. This takes any piece of lost technology and 'switches it off', rendering it inert, harmless and incapable of venting dimension-binding energy into your location. A cold shutdown for anything techy.
Shapeshifting- Yuuno's party trick. This is his most famous ability- he can transform from a human into a ferret. He seems to be one of the few characters, if the only, with the mastery of this ability, but it's not without its social stigmas. He uses this ability to conserve on space in living quarters because he can just sleep on a table or something. It also allows him to lick Nanoha's face and get away with it. However, because most humanoids who can turn into animals are 'familiars'- magically created beings that serve a specific purpose and are bound to their masters- many people mistake him for Nanoha's familiar and not an independant being. Chrono Harlaown delights as referring to Yuuno as Nanoha's familiar on official documents, which he loathes.
Physical Healing- Yuuno's your healplz. He is exceptionally good at this, and although it might take some time, he can heal pretty much any wound on a single person. He doesn't even need to be around for it- he can set up a heal, and then leave to do something completely different and as long as someone doesn't leave the bubble he's set up, they will heal perfectly. He's used this in the past even to heal Nanoha after someone drained most of the energy from her 'linker core' organ, which is the equivalent of a vampire sucking most of your blood out of you.
Temporal Forcefield- Ah, now this one's a doozy. Yuuno can use an ability which screws with the flow of time in a localised area, masking what's happening from other people by changing how observers percieve time. Everyone drops a little out of sync with local timeflow. Certain people who can acknowledge the flow of time probably get a wicked contact buzz.
Floater Field- More a footnote than anything else. Yuuno can create an antigravity field by placing a circle underneath someone. More fields equals more floating. This is often used to save people from hitting the ground and dying. Like Nanoha.
Dispel Forcefield- Ah, now this one's a doozy. Yuuno can destroy forcefields, even exceptionally well-made ones, especially if he has help. It's like how a hacker enters a computer- devote enough time and effort and power to it, and he can brute force through.
Transporter/Dimensional Teleport/Transporter High- Another doozy. This allows Yuuno to teleport other people, or multiple people at once. He used it to zap Nanoha out of a spaceship inbetween dimensions, and also to transport a monster from the ocean TO EARTH'S ORBIT. Do not screw with Yuuno, he will blink you into outer space.
Chain Bind- Yuuno can make chains at a moment's notice. These are fancy green energy chains. They will lock you down, and be warned- they're an offensive spell, and pull hard enough on them and whatever they're grabbing into will be cut apart.
Struggle Bind- Another surprisingly powerful ability. Much like the other one, Yuuno creates an energy chain- but these are so powerful they can dispel things and dissolve transformations. It causes spells to stop working and grounds magical energy. This is a very powerful trick.
It's worth nothing that Yuuno can also combine these together using 'arias', which do things like create a very strong forcefield that heals anything inside it.