It will vary from game to game. For some it's just DLC like Marvel Vs. Capcom and Wipeout. For others like Motorstorm RC, Top Darts and I believe Hustle Kings you just buy the game on one console and get it for free on the other.
Does it mean i will only have to buy DLC once or does it mean i only have to buy one copy of a game?
I didn´t get it: so, for instance, I have a Wipeout 2048 cartridge for PS Vita - how do I go about cross playing with the PS3, since I don´t have Wipeout HD installed on my PS3? If I don't have to buy the game all over again for my PS3, how does it work?
Cross platform saves : 2 separate games, one for vita one for ps3. You move the save between consoles to continue. Example ruin/warriors lair and MLB 2012.
Cross platform multiplayer : 2 separate games, one for vita one for ps3. In this case a person playing a multiplayer game can play against someone else playing the game on ps3. Wipeout and hustle kings are examples.
Remote play : game runs on ps3 but instead of using a tv for view and dual shock 3 for control, the vita is used via a wifi connection. Very little support for this now, just psone games, pixel junk.
Sharing dlc. Across platform is separate from these and up to publisher to allow or not, but it is more likely to occur with cross platform save and multiplayer games to allow the cross play functions to work.
My question is this can you stop in the middle of a game save it then pick it up on the Vita? It seems like this only allows a season to be saved not a game
Nope, only in between games.
Motorstorm RC does this because it stores the data on their servers and synchornises when you boot up the game on either console.