Evidence generally falls into four categories:
Real evidence is any actual object that was directly involved in an event in the case. It could be the weapon used to murder a victim, like a gun or a hammer, or the tool used to break into a house, like a crowbar. Demonstrative evidence, on the other hand, is an illustration of evidence -- something like a map of the crime scene. Documentary evidence, also a type of real evidence, describes letters, contracts, newspapers or anything that contains human language. Testimonial, or anecdotal evidence, is oral or written evidence from victims, suspects and witnesses involved with the case.
All of the above types of evidence need to follow the rules of admissibility, which are there to make sure that anything introduced to the court as evidence meets three criteria:
Relevant evidence proves or disproves a fact of a crime, but it doesn't necessarily prove anyone's guilt or innocence -- it's simply a broad term that describes any piece of evidence related to the case. A tool stained with a suspect's blood might be relevant, for example, but so is the person who sold that tool to the suspect. However, testimony from a toddler discussing a broken house contract would be deemed irrelevant and therefore inadmissible because a child would be too young to understand the case.
Material evidence, on the other hand, needs to prove an essential fact of the case. For example, if a lawyer attempts to prove that the drapes in the room of a murder scene were blue, chances are a judge will deem such evidence immaterial. The room itself may be relevant to the case, but it's likely the color of the drapes doesn't have anything to do with the murder. Finally, competent evidence is an object or testimony proven to be reliable, like matching fingerprints, the results of a DNA test, or an expert on footwear impressions. An expert giving an opinion that isn't generally accepted in his field, on the other hand, is neither competent nor admissible.
There's an endless amount of inconsistency and several exceptions, but one of the most important rules of evidence is the hearsay rule. This rule prohibits secondhand testimony, or evidence of the "he said, she said" variety, during a trial. If an eyewitness to an accident tells his friend the details after the event, the eyewitness's friend's testimony would be hearsay and considered inadmissible.
A judge can dismiss evidence for several other reasons. A presentation will take an unnecessarily long time; upsetting photographs will unfairly incite a jury, or forensics experts might have gathered evidence illegally. However, the main reason for declaring inadmissibility is to make sure evidence is reliable and fair to both sides of a case. For lots more information on crime and the legal system, see the next page.