A jury is a body of citizens in an common law jurisdiction which decides questions of fact. The term can refer to a grand jury which issues an indictment or a petite jury which is present at trial. When used alone the term usually refers to the latter.
In the United States defendants in most serious criminal cases have a right to trial by jury, although in practice most criminal actions are resolved by plea bargain.
In most civil law systems, a jury is not used, although lay assessors perform many of the functions of juries in some systems. In Japan, the post-war constitution guarantees a right to trial by jury although this right is almost never used because it is believed that anyone who is desperate enough to want to have fellow citizens decide their fate is almost certainly guilty of the crime that they are charged with.