Armor is what is going to protect you from a monster's wrath. Armor boosts your defense well above its natural 50, which will allow you to take a few hits and give you the safety that you need to go embark on quests. Armor is very versatile.
Complete sets will give you not only the look of the monster, but also sets of skills, and elemental resistances and weaknesses that match the monster. For example, a full set of Jaggi armor will make you incredibly weak to fire, just like Great Jaggi is weak to fire.
Mixing and matching sets can offset this, and provide an incredibly high level of customization and fine tuning.
Unlike weapons, you cannot ever upgrade your armor using monster materials into a different piece of armor. But you can increase the defense slightly through odd items called Armor Spheres. They can be found as quest rewards and from mining nodes. Armor Spheres come in a variety of different levels. To upgrade armor, you will need different types of Armor Spheres plus material points . Every monster part gives a certain number of points. Use a variety of monster parts to fulfill the point requirement, and then poof, upgraded armor!
Only the hard, raw defense will be boosted. Elemental weaknesses will always stay the same. Remedy this by eating for elemental resistance at the kitchen, or by having multiple sets of armor for different purposes.
In previous games, you could eat away negative elemental resistances and have it always boost to +3 no matter how pitiful it was, but no longer in this game! Eating for an elemental resistance will increase that resistance by a set amount, so you could still be deep in negative resistance after your meal.
Armor from deviant monsters is different and will not use Armor Spheres. Rather, they require the tickets you get from different levels of the quest, plus monster parts.