Informal is talking more casually. We should only talk informally to our friends, Family and people we know really well. Formal is talking more politely to people. We should talk formally to people that are important for example. The prime minister, A Boss.
The things that are different about them is that formal talking is that they say words very differently. For example formal talk uses words like develop instead of make machinery instead of machine. These words are from Tractors. Informal words use words like better instead of stronger and make instead of produce. These words are from Tysgaru Shamisen.
Informal is talking more casually. Formal is talking more politely to people.