Raise DEF skill allows units to get additional defense. This helps them take less damage from enemies' attacks.
Leader Skills boost any unit on the team who meets the specified condition. If the condition is not met, the unit will not receive any boost.
Super Attacks usually boost units' defense for a specified amount of turns. If it's not specified, their defense boosts will stack indefinitely.
Units with defense rising Passive Skills either boost just themselves, or allies in the same turn.