Qui tam actions are those where you bring an action against another party on behalf of the government. So, in a qui tam action, you are taking the case to court, but the government is the plaintiff. You may do this because you will receive part of the damages if you win the case.
Qui tam actions relate to whistleblowers. The National Whistleblower Center explains through this type of action, you can sue someone for committing fraud against the government.
You can file
You can file a qui tam action when the government does not wish to pursue the case. You cannot do it if the government is filing a case on its own. However, if you have evidence and the government is not going to court, then you have the right to file.
Acting as a relator
When you file a qui tam action, you become a relator. Realtor is basically the same as a whistleblower. Essentially, you have evidence someone is committing fraud against a government program, such as getting benefits to which they have no right or submitting false information on official documents. You do have to file the action and take all the steps to prove your case.
Remember that when you file a qui tam action, you do have some benefits. Not only can you help to fight fraud within the government that ends up hurting everyone, but you also have a stake in any damages the court makes the other party pay. If you lose the case, then you are only out the costs of taking the action to court.