The difference between expert and novice game players is the level of expertise. An expert player is one who can play a game very well – without dying, without losing any given advantages, getting high scores, passing through levels, beating games, etc. A novice player is one who can play a game fairly well. Their level of playing is not as high as an expert player, but they do not die often, nor do they get high scores, they can pass through levels, but they might not be able to win a game in its entirety.
The reason why these two definitions can overlap is that each person might be better at a specific type of video game. For example, a person may be an expert at first-person shooter games, but may be a novice player in platform games. It can also be relative to other players. There is not a defining line between these two types of players, but there is a general thought about the differences.