After criminal accusations, a lot of different elements may impact the case. They could include the testimony of the accused, the statements from police officers, the forensic evidence from the scene and much more. All of these work together to help the judge and jury determine the outcome of the case.
One key point, in many cases, is the eyewitness testimony. When a witness comes to court and tells the jury what they saw, it holds a lot of weight. After all, that witness may have nothing to do with the case otherwise. They have nothing to gain by lying. The jury often assumes that they're telling the truth.