i really never got primary typer VS. secondary type....is there really a difference?
There actually is. I remember Mario with Lasers explaining it once...
Take Garchomp. It's a Dragon/Ground-type. For a minute, just look at the type. Since it's Dragon, and
then Ground, we'd assume that it's a Dragon Pokemon with with characteristics of a Ground-type (it can't fly, it's classified as a 'Land Shark', it lives in caves, etc.). Look at Garchomp's design--it's indeed a Dragon with Ground characteristics.
Consider if it's typing was reversed to Ground/Dragon. That would mean that Garchomp is primarily a Ground-type with the characteristics of a Dragon. In other words, it'd look a lot like Onix--primarily something from the earth, but big, long, and fearsome, like a dragon.
See the difference? The Primary type is dictated by it's basis as a Pokemon. It's secondary type is usually decided by other elemental characteristics the Pokemon has.