There are basically two types of witnesses. There are people without special, professional qualifications that have relevant information regarding the case. Those are called "lay" witnesses. The second kind of witness is an expert witness. An expert witness has knowledge beyond that of a "lay" witness and can provide opinion testimony. Ordinarily lay witnesses cannot provide testimony regarding their own opinion regarding what happened unless it relates to matters of common knowledge, such as his or her opinion regarding speed of a vehicle, how far one could see down the roadway, or other matters of common knowledge. An expert witness , on the other hand, can provide his or her own opinion regarding how an incident took place (accident reconstruction expert); what the standard of care would have been for a physician (medical expert); or how a building should have been constructed (engineering expert). There are also other expert witnesses commonly used such as vocational rehabilitation experts and economists who testify regarding the job the injured person would have been in had the accident not happened, the job they could be in given the disabilities the accident has caused and what the economic result of that is in today's dollars.
All witnesses are to be evaluated on the basis of whether or not in the mind of the jurors they should be believed. The fact that somebody is an expert witness does not make them automatically more believable on a particular topic if the jury doesn't find their testimony and explanations convincing.