Both documentary and testimonial evidence must be as free from hearsay as possible. Hearsay is when someone else told you the fact that you are testifying to. For instance, a witness would not be allowed to say that although they did not see the accused rob the bank, they were told by someone else, that that person had seen the individual rob the bank. The best person to testify, and in fact the only person who would be allowed to testify, would be the individual who actually saw the individual robbing the bank. Similarly, a document that purports to reveal not a fact, but a hearsay fact, may be admissible for other reasons because not all of it contains hearsay, but insofar as it purports to establish facts that are not within the knowledge of the creator of the document, it may go to weight, and may mean that the document has no evidentiary value.
Hearsay is admissible however.
There are many exceptions at Common Law that permit hearsay to be admitted, but these exceptions and the circumstances in which they apply are sufficient to fill several legal texts. Opinion evidence usually emerges from the testimony of a witness. One does not have to be an expert to express opinion evidence. When a witness testifies as to things that would normally be within the knowledge of an individual, such as the speed of a motor vehicle or the general health of an individual, these things don't require any special training or knowledge. Where, however, the authorities seek to call someone to express an expert opinion on something that is not within the general realm of knowledge, then that person must be qualified as an expert before the court will allow them to give the opinion.