Nobody likes to lose. And most people would like a second bite at the apple. The appeal of an appeal is that it gives you a second shot. But that second bite could cost even more than the first and not prove any more successful, so consider well.
TL;DR (Too Long; Didn't Read)
People who lose a case often take appeals in order to get another chance before a different tribunal, to establish that their interpretations of the law were correct. In addition, a party has a limited time to file an appeal and may lose the chance if action isn't taken.
The Appeal Process
In most cases in the trial courts, the losing party has the right to appeal to the next highest tribunal. In California, for example, superior courts are trial courts, and appeal is taken to the Court of Appeals. In federal courts, you can appeal a decision of the trial court, called the district court, to the Circuit Court of Appeals.
Who can appeal? Any party who loses in trial court can appeal in civil court. In criminal court, the defendant can appeal a guilty verdict, but the prosecutor cannot appeal if the defendant is found innocent. Either side in criminal court can appeal the sentence imposed.
The Issues on Appeal
Generally, appellate courts consider only arguments about the law, either interpretations of a statute or other document or the application of the law to a particular set of facts. The lower courts are the finders of fact, in both jury trials and court trials, and the appellate court will not reweigh the evidence. It will assume that the trial court correctly decided the facts unless a factual finding is clearly against the weight of the evidence.
Because of this distinction, appeals are very different from trials. They have no witnesses, present no new evidence and display no smoking guns. The appeal is heard on the written record from the trial, and arguments are presented in the form of legal briefs. The appellate court may or may not permit an oral argument in which the attorneys each present their positions to the court in person.
Reasons to Appeal
Many people want to file an appeal because they believe that they are right, that the other side is wrong and that the lower court simply got the facts wrong. However, this ignores the reality that appellate courts do not usually address the facts at all, but accept the lower court findings.
Generally, attorneys will not agree that an appeal is warranted unless there are clear questions of law or legal interpretation that they believe were wrongly decided in the trial court. If the trial court judge made serious mistakes that could have influenced the outcome of the case or ignored current law, an appeal should be considered. Remember not to nick-pick here, since "harmless errors," those unlikely to make a substantial impact on the result at trial, cannot be grounds for reversal on appeal. Typical judicial mistakes that might justify an appeal include refusing essential evidence, refusing to allow expert testimony, giving the jury the wrong instructions or showing bias.
One factor that weighs into every decision to appeal is the fact that it is a now-or-never decision. In every state and federal court, you have a certain amount of time after a judgment is entered to file an appeal, usually 30 days. If you don't file during those 30 days, you can never file. The thought that you might regret not appealing provides a strong incentive to appeal and give it a shot.