The Oxford Dictionary defines the word ‘adversary’ as ‘one’s opponent in a contest, conflict, or dispute’. That definition goes some way to explaining the adversarial legal system in the England and Wales under which, essentially, representatives from each party take opposing positions to debate and argue their case, whilst the Judge's role is to uphold principles of fairness and equality and to remain neutral until the very end when he gives judgment. This contrasts with the inquisitorial legal system (commonly found in civil law countries e.g. France / Italy) which sees the Judge take a much more active role in preparing evidence, questioning witnesses and finding the truth.
In an adversarial legal system, previous decisions made by higher Courts form a precedent which will bind the lower Courts. In contrast, Judges in an inquisitorial legal system tend to be free to make decisions on a case-by-case basis.
The principle behind the adversarial legal system is to place distance between the investigation taking place and the person who ultimately decides the outcome. The system empowers the parties to the dispute to take control of their own case on the basis that they (as opposed to a judge) are better placed to present their best case.
However, even though an English Judge may not decide what matters to investigate and how to do so, his role is by no means passive. Under the Civil Procedure Rules ("CPR") which came into force in 1999, the Court has very wide case management powers which are used to ensure that the dispute is resolved efficiently and in accordance with the CPR’s overriding objective of enabling the Court to deal with cases justly and at a proportionate cost. The Court will do so by excluding superfluous evidence, managing the parties' costs, and setting a strict timetable to Trial under threat of sanction should any of the dates be missed.
There is, however, a perceived unfairness in the adversarial legal system in situations where the parties do not have 'equality of arms'; a better resourced party may be more able to gather evidence and present a stronger case to the Judge than their opposition. Furthermore, because the parties have near complete conduct of the case from start to judgment, they are able to choose what evidence they put before the Court. In comparison, in an inquisitorial system the Judge is involved throughout the process and actually steers the collation and preparation of evidence. He is therefore able to decide what evidence is admitted by both parties, before questioning the witnesses himself and going on to make an informed decision on the outcome.
That said, given the importance placed on the investigative role of an inquisitorial Judge, the risk of bias is (arguably) greater in an inquisitorial system.