Defamation is language that injures the good name or reputation of another person.
If the statements complained about are designed to be read, its libel. Slander involves the spoken word. Unless it's on TV or radio, then it goes back to the libel category.
Defamatory language must tend to lower the victim in the esteem of any substantial and respectable group, even though it can be a small minority. The person complaining cannot rely on an artificial, unreasonable or tortured interpretation of what was said. It can't be something only "supersensitive persons" would consider slanderous.
A statement is not actionable if it is substantially true. Defamation can not be claimed where there the speaker has some privilege. For example, statements made in the course of litigation are protected. Statements of fact are actionable, expressions of opinion are not.
There are two types of slander.
Under slander per se, no proof of damages is required, where the spoken words impute crime, loathsome disease, relate to a person's business or profession or go to such other basic characteristics.
In other instances, or slander per quod, then money damages must be proved.
Defamation law is complex. Volumes have been written. The reason that defamation presents problems for the courts is that they must balance two basic rights. One is our right to freedom of speech. The other is the individual's right to protection against harm to reputation.
For example, where the person complaining that they've been defamed is a public figure, then they must prove that the harmful statements were made with "knowledge of falsehood or reckless disregard for the truth." That means that people including politicians, actors and others in the public eye can not recover for defamation unless they prove that defamatory statements published against them were intentional or wantonly reckless.