Evidence
In criminal trials, the evidence must prove that the defendant committed every element of the crime beyond a reasonable doubt. The entire burden of proof is on the prosecution. A defendant is always presumed to be not guilty and is never required to present evidence of his innocence.
Evidence is either direct or circumstantial. Most crimes are proved through circumstantial evidence that "connects the dots" from the crime to the defendant. If the gun the defendant owned is the same type with which the victim was shot, the gun will be admitted into evidence. If the defendant and the victim were in a fistfight witnessed by neighbors, that testimony is admissible to show a motive existed for a later assault. These facts may be enough for a jury to convict the defendant. Direct evidence, such as that of an eyewitness, rarely exists and convictions are generally based on the circumstantial evidence that the prosecution offers at trial.
Evidence can be almost anything: testimony, videotapes, clothing, reports, expert opinions, business records, etc. However, even the most persuasive testimony or item will not be presented to the jury unless it is "admissible."