My view of synergy is using skills that can act perfectly on their own, together so they enhanche eachother. And my view of a combo is having to use a number of skills directly after eachother in order to gain an effect.
A perfect example of synergy is the Curse + Elemental Strike thing. You can use Curse perfectly on its own and then hack away with a sword for example. And you can use Elemental Strike for multiple times to kill someone too. But you can use Curse to enhance the effect of Elemental Strike, so you will need less Elemental Strikes to kill your enemy.
An example of a simple combo would someone with dual wielding weapons: you start with a lead attack (which is skill 1), you continue with an attack from the weapon in your other hand (skill 2), you block a counterattack with both your weapons (skill 3) and then comes the finishing move with also both your weapons (skill 4).
This means that when one skill in the combo fails, the whole combo will fail (the blocking skill 3 fails, so you get hit and can't use your finishing move). So skill synergy gives you a lot more freedom on how you want to play the game, what skills you want to use and how you want to use them. A combo gives you a smaller range of things you can do with certain skills. Also, a combo makes me think more of hack&slash games.