It's all about the probability you want to express. Likely and probably are sort of similar and they mean that the probability of something happening is quite high. Possible and perhaps are used when you're less certain of something. For example, let's say "Fast Guy" has to compete with "Slow Guy" in a running contest. Obviously you'd say something like: You: "It's likely that Fast Guy will win. / Fast guy will probably win." However, if they'd have to compete in a chess match and let's say that you don't know who will win, a conversation might go like this: Someone: "I think Fast Guy will win!" You: "That's possible, but perhaps Slow Guy will win this one." So it's more like an option whereas "likely/probably" mean that it's to be expected.