Possibly - It's possible that something will happen (0% < probability <=100%) Probably/Likely - It's probable that something will happen (50% < Probability <= 100%) Perhaps/Maybe - These are related to certainty more than probability. They just mean that it is uncertain if something will happen or not. 'Maybe' is more colloquial. If someone asks you a yes-no question, responding with "I guess" means "Maybe, but I don't really care either way." "I guess so" means "I think so, but I'm not certain". Note that these can only be responses to questions, and are NOT adverbs like the others. You should also remember that they're very colloquial.

I probably use those words at least once a day, possibly more. Maybe I shouldn't use them so often, but I guess you understand now perhaps?

Everything said above is true. I'd also note that the impression given by these words depends a lot on context - the topic being discussed, other qualifiers being used, even the tone of voice (if it is spoken English). "Pooooooooossibly...?" has a different meaning from "Yes, possibly so!" for example. So (as with a lot of these kinds of expressions) the exact meaning can be a bit "case by case". Just to make it extra hard for language learners!