Prosecutors and defense attorneys build their cases from the evidence that is available to them. Although individual pieces of evidence may be categorized in various ways, there is one standard that all evidence must meet: the standard of relevancy.
Evidence is said to be relevant if it makes a fact more or less likely to be true than it would be without that evidence. For example, the testimony of a witness who says she watched the defendant kill the victim would be considered relevant evidence because her testimony advances the argument that the defendant is guilty of murder.
Not all evidence is as clearly relevant as an eyewitness testimony, however. For instance, the fact that the defendant purchased a new handgun shortly before the murder occurred could be considered relevant; however, if the defense showed that the defendant was a handgun enthusiast or firearm collector, the fact that he had recently purchased a new gun would suddenly become less relevant in the murder case.
Although only relevant evidence may be admissible in a trial, it is not the case that all relevant evidence will be admissible. Sometimes, a judge may exclude relevant evidence based on some other evidence rule.
For example, if a judge determines that a piece of evidence is likely to arouse the jurors’ emotions to an unreasonable degree, that evidence may be deemed inadmissible, even if it meets the standard of relevancy.