To answer the first question, there are generally two accepted standards – the fry standard, and the Daubert standard.

The first major legal argument as to admitting evidence was centered on the use of polygraph machines in 1923. It’s centered around the scientific validity of the lie detector machine. The decision in this case established the Frye standard.
It demonstrated that the admitting expert testimony produced from a well recognized, scientific application or discovery is the thing from which the deduction is made, and that it must be sufficiently established have gained general acceptance in the field.

The Daubert standard was established in 1993 in which, the court ruled that the federal standard includes general acceptance, but also looks at the science and it’s application. And therefore, if there is a journal acceptance of the theory has applied testing that has been validated, it may have sufficient evidence to be admissible in court.

So to answer the second question, it is someone who is qualified as an expert by knowledge, skill, training, or education. Generally speaking, the experts, scientific, technical and specific knowledge will help those understand the evidence or to determine affect an issue. Typically a testimony is a product based on reliable principles and methods, and that those are applied to the facts of the case.

Therefore of the majority of states follow the Daubert standard for expert witnesses. It summarizes that experts, who are qualified by knowledge, skill, experience, training, or education can testify in a case. Therefore, to be considered an expert, they must be a general acceptance of that individuals, level of training and experience.
Expert witnesses are generally required to review documents, perform research, sometimes conduct an investigation in terms of gathering additional date, and finally form an opinion, and provide that orally as well as in writing to counsel.

It goes without saying that experts should demonstrate confidence in their field, excellent communication, skills, reliability, and perhaps, most importantly honesty. These are traits that are most desirable to counsel.