Suikoden Tactics takes place at the same time as
Suikoden IV. But instead of following Snoe and your character from that game, you take control of a small band of adventurers trying to find the secret behind a series of magical weapons called Rune Cannons.
Your main character is Kyril who has inherited the quest from his father who died when transformed into an unearthly monster by the Rune Cannons. During your quest, you will run into several characters and events that appear in Suikoden IV, but by no means is it necessary to have played the other game in order to have fun with this one.
Suikoden Tactics is, like I said above, a strategy RPG in the same vein as FFT and Disgaea: Hour of Darkness. Each battle is broken up into a series of rounds. Each round will have you moving your characters and attacking one at a time. But instead of simply moving from enemy to enemy attacking them from the side or behind, Suikoden gives you many other moves you can perform.
For instance, you can move next to another one of your teammates and talk to that person instead of fighting. This will typically increase the fighter's goodwill towards each other. If the two fighters have enough goodwill, then they will start to watch each other's backs. This means that you will see one teammate throw him/her/itself in front of an attack if they are positioned appropriately. Other actions you will see if the two characters like each other (which is essentially what the goodwill mechanic is) are characters joining in attacks and being able to execute powerful co-op maneuvers that will typically hit multiple enemies.
Other moves include using runes to either mark territory or attack opponents. If you use a rune, then the tiles that the move touched are then colored to that rune (purple for lightning, red for fire, blue for water, etc). These colored tiles will either help or hinder characters as they stand on them. These alterations typically include either a health boost or hit after the round is complete.