Defamation and libel both involve someone spreading false information about you that damages your reputation. All libel is defamation, but all defamation is not libel.
Defamation is the overall category of this type of crime, and libel is a type of defamation.
Defamation includes all activities where someone makes untrue statements about you that could cause you harm. It has two types, libel and slander.
Libel is the type of defamation that occurs when someone writes something harmful about you that they cannot back up with evidence to prove it is true. Once something is in writing, such as a transcript, it becomes libel.
Slander is the type of defamation that occurs when someone else speaks false things about you that could harm your reputation.
Defamation of any kind requires intent to cause harm and to impact your reputation. If someone presents what they write or say about you as an opinion, then it is not defamation. There must be a presentation that this rumor or claim is factual and true to rise to the level of defamation.
The First Amendment cannot protect someone from defamation charges if they were legitimately spreading untrue statements about you that harmed your reputation. Once it crosses over the line and causes damage to you, then it becomes criminal and not a right.
The definition of defamation and libel here is very general. There are exceptions and different situations where one party may not be liable for the crime, which can make a defamation case tricky.