Well the most basic thing about synergy combos is this: you or someone else has a skill that other people can use along with it to make it stronger. For instance, sorcs have a summon atronach skill that once used, another nearby player can hit 'x' (by default) and make the damage done more powerful. There are tons of these skills in the game, and when you can use one, a pop-up will appear on your screen telling you to hit 'x'.
I know about those synergies thank you tho it's still good info, I was wondering about the ones you use two moves together with another player. Like I read about a rogues type trap and a sorcerers fire spell together make a big aoe of fire.