Hearsay is an out-of-court statement offered to prove the truth of whatever it asserts.
Hearsay evidence is often inadmissible at trial. However, many exclusions and exceptions exist.
For something to be hearsay, it does not matter whether the statement was oral or written. Generally speaking, hearsay cannot be used as evidence at trial.
Federal Rules of Evidence (FRE)
Rule 801(c) of the FRE defines hearsay. Rule 802 prohibits the admissibility of hearsay. Rule 803, Rule 804 and Rule 807 list exceptions to the rule against hearsay. Rule 805 discusses hearsay within hearsay. Rule 806 discusses the credibility of the declarant of the hearsay.
Although hearsay rules differ by jurisdiction, many state hearsay rules are modeled on the Federal Rules of Evidence.