There are a total of 90 different treasures in the game. Ways for obtaining them are different - simple items can be bought from shop, more rare ones can be found in advanced shop, very powerful artifacts are captured after winning fight agains the ruler of any kingdom (each ruler has special artifact), some items can be got from exploring dungeons, there is also a possibility to make them by using a special action from a list of strategies.
Treasures can be used for a huge variety of things - for summoning of units, for using spells or strategies, for diplomacy with other nations/traders/dark temple. And also any unit can equip those treasures to gain the benefits, they give.
Ctrl - increases maximum number of units in the squad that unit having the skill is the leader of.
Atk, Def, Hit, Spd, Mgc - increases specified parameters of the unit.
Skill1, Skill2 - skills that unit will get after equipping the treasure (you can read their description on skill's page).