In Testifying as an expert witness in computer crimes cases, Deb Shinder writes:
When we think about witness testimony, most of us think first of material fact witnesses (lay witnesses). These are persons who have first-hand knowledge of matters relating to a particular case. For example, if you as an IT professional observed child pornography on the computer of one of your users at work, you could testify to this as a material witness.
An expert witness, on the other hand, is not involved in the case at hand, but has special knowledge and expertise pertaining to the subject matter of the case. For example, if you are recognized as an expert in the subject of malicious software, you could testify as to whether and how a malware infection could cause illegal pornographic images to be downloaded to a person’s computer without his or her knowledge. A major difference between material fact witnesses and expert witnesses is that the former are not generally allowed to give opinions or draw conclusions, whereas experts are. Another difference is that expert witnesses are generally paid for their testimony (by the prosecution or defense, depending on the side for which they’re testifying).
Read more: techrepublic.com.