Hearsay is an out-of-court statement offered for the truth of the matter asserted, i.e. the truth of the content of the statement. For example the statement: "I told her the light was green," if offered to prove the light was green, would be hearsay. If offered for another reason, such as to prove the speaker knows English, it might not.
Is it a statement?
Only statements of fact fall under the rule. Questions, instructions and commands do not.
Is it offered for the truth of the matter asserted?
As in the above example, the statement: "I told her the light was green," if offered to prove the light was green, would be hearsay. If offered for another reason, such as to prove the speaker knows English, or to show the listener had knowledge of the light's color, then it would not be hearsay.
Is it exempt from the hearsay rule?
Certain statements which ARE out-of-court statements offered for the truth of the matter are exempt from the hearsay rule and are NOT considered hearsay. (Federal Rule 801). The classic example is a statement made by a party opponent (i.e. Plaintiff offers Defendant's statement into evidence in a civil case, or government offers criminal Defendant's statement into evidence).
Does it meet an exception to hearsay?
Some statements are hearsay but they are admissible anyways because they meet an exception (Federal Rules 803 and 804). Classic examples are dying declarations (in murder cases), excited utterances, business or public records, and statemetns made for the purpose of medical diagnosis.
Does the Sixth Amendment apply?
In a criminal case, an out-of-court statement may be admissible under the hearsay rules, but still not be admissible under the Constitution. If an out-of-court statement of a witness is offered against a defendant and is found to be "testimonial," it will not be allowed in at trial unless the witness is 1) available to be cross-examined at trial or 2) was available at a prior hearing for cross-examination and unavailable now.