A combo deck is a deck that relies on a specific combination of cards to win the game. The cards in a combo deck are chosen to work together to create a powerful effect that should either win immediately, or create so much value that you in effect win the game. A two-card combo is the simplest form of a combo, but some combo decks may require three or more cards to work together.
The original Orbs CCG game had a combo deck that aimed to cast a bunch of direct damage and energy gaining cards all on one turn to win the game. Since the new version of Orbs CCG has some different cards, that deck is no longer possible, but that doesn't mean that combo decks won't appear in the future!
Synergy is when two cards work well together, but don't necessarily win the game on the spot. For example, Mind Goblin has synergy with Nimble cards like Tree Nuts, because Mind Goblin's triggered ability lets you draw a card when you play Tree Nuts.
Combo is different because the reward for playing the cards together is so high that it should basically win the game. Drawing a card is not quite good enough of a payoff to be considered a combo, but it is considered synergy.