For a statement to be found legally defamatory, it must prove a defamatory statement was made, published to a third party, the person making the statement knew or was negligent in determining the truth, and dangers arose.
The key issues are the statement and the truth aspects. If a statement is in the public, it has been published somehow, spoken or written and accessible to the public. Damages are a matter for trial, in proving if and to what extent a person was injured by the statement.
When making a statement, to be classified as defamatory, the statement must have the tendency to harm the reputation of the person subject to the statement. In other words, if you say that John down the street is a great guy, even if false, the statement is not defamatory. Now if you say Janet down the street eats puppies, that would be more likely to be defamatory - depending on context.
As for the negligence in regards to the truth, it is a bit of a difficult standard. The requirement is that the person making the statement did not take reasonable care in ensuring the statement was true. This tends to be more about not spreading rumors. Or in the example of a woman eating puppies, it is an outrageous statement. Passing this statement on would be negligent, there is no way to verify the statement and it easily could have been made up.
A note, if something is true and conveyed in a statement, the truth of the statement is an absolute defense. If you say that someone does not follow the law because they have been arrested, if any of that statement is true then the statement is not defamatory.
Defamation is difficult to prove. Establishing reputation before and after a statement can be a challenge. Additionally, proving monetary damages can be hard to say the least. Still it is possible and just because it is difficult to prove does not mean impossible. Different facts and circumstances can make the charge more or less difficult to prove. Be cautious and know defamation is an issue.
As a final note, defamation tends to be dealt with at the state level. While these tests are fairly consistent, there may be differences or alternative tests depending on where the incident took place and the relevant jurisdiction.