Every single armor piece has a unique spread of skill points attached to it. View your skills from the START menu, and press Y when on a skill to see a detail of what it does, and the points necessary. Most skills will need ten skill points, but some higher levels of skills will need fifteen or twenty points.
It is possible to dip below zero! Negative skill points will give you a very bad version of the skill. For example, Sharpness+10 gives the amazing skill Razor Sharp, which halves the rate your weapon will lose sharpness. But Sharpness-10 gives the terrible Blunt Edge, which doubles the rate!
If you have +9 skill points, you won't have the skill, and if you have -9 skill points, you won't have the bad skill either. This means when mixing and matching pieces that you can go as low as -9 skill points and still suffer no consequence.
Wearing a complete set of monster armor will usually give you a small set of related skills, some that might relate to the monster it was made from, and one or two negative skills that you can use Decorations and Talismans to take away.
There is an immense variety in the types of armor skills. Some are Gunner-specific (only found in Gunner armor) and others are Blademaster-specific. Some will affect how your stamina is managed, how fast you can take a Potion or use items, and others can increase your Attack, Defense, and Health caps, how your weapon sharpness is managed, increase damage from specific types of ammo, and so, so much more.