Anike has explained the difference above.
To add to her answer: In your question you ask 'what situation to use' these terms. We use these phrases in informal speech, so when talking to a friend or someone in a situation where you are both relaxed and talking in a friendly way. Sometimes people will be offended if you say "Get it?" because they think it sounds quite aggressive/too direct. This varies in different cultures (so in different English speaking countries/organisations/groups of people), but generally you would not use 'get it/got it' in any situation where you want to be more formal (like a job interview for example).