Buff and Debuff are two common words used to explain status effects on Heroes or enemies.
Buff is any effect that increases the performance or statistics of the affected target. For example: Lady Jin's skill 'Inspiration' increases allies' attack for several rounds.
Debuff is any effect that decreases the performance or statistics of the affected target. For example: Gerhard the Grim's skill 'Harsh Judgment' affects enemies' mana points and reduces their attack for several rounds.
NOTE: Buffs and Debuffs and their duration are shown on the affected target. You can also view how many turns the effect has left by tapping and holding the affected target.
Status Effects affect Heroes in multiple ways. Much like a Debuff, they have negative consequences for the affected Hero. For example: Troll enemies throw stones that may temporarily block some of your Runes.