A more thorough explanation:

An accusation is when someone says that another person has done something wrong, like breaking the law or doing something bad. It can be said informally, like when a friend accuses another friend of stealing something, or it can be more formal, like when a prosecutor charges someone with a crime.

• Informal: "I accuse you of taking my phone."
• Formal: A prosecutor accuses someone of murder and brings them to trial.

The first example is informal because it's just one person accusing another person of something. The second example is more formal because it involves the legal system and a prosecutor charging someone with a crime.