When the number of party members reaches 4, the formation option becomes accessible.
Formations have various effects which can increase the party's attack, defense, or even resistance to abnormal status to advance the battles to your advantage.
Formations also give the party the ability to unleash certain special attacks known as tactics.
As you advance in the main story, you will get new formations and tactics.
Besides normal skills, auto-skills can be learned using the Skill Books that can be obtained from enemies.
For example, there are auto-skills that get you more EXP and gold, and each auto-skill has a rank from C to S.

Skill Books
Find Skill Books scattered through the game world to learn auto-skills that can be used in battles.
There are super useful effects such as double attacks, halved MP consumption, etc. so take advantage of the skills and grow your characters as you like!