Contracts are meant to provide protections for both parties that sign them. Each has conditions that they must meet. When either side doesn't uphold their end of the contract, a breach of contract has occurred. In some cases, this might lead to a lawsuit.
One important thing to consider in these cases is that not all breach of contract cases mean that one party is going to receive a monetary award. The remedy for the matter must be appropriate for the type of breach.
There are two broad categories of breaches -- material and immaterial. A material breach is one in which it is assumed that there are damages. An immaterial breach is one in which there likely aren't any actual damages. It might be possible to receive damages for an immaterial breach, but you would have to show that you suffered damages because of the breach.
Many breach of contract cases are handled privately between the parties. Coming to a mutual agreement is usually the preferred resolution since it is often the fastest and least costly route. If that isn't possible, you might use alternative dispute resolution or head to court. The party who committed the breach is the defendant. The party who is claiming damages is the complainant.
Regardless of how you plan to handle a breach of contract case, you need to weigh your options carefully. Think about the costs of your options versus the potential outcomes. This can give you an idea of whether you should pursue things beyond a mutual agreement. An experienced attorney can provide you with guidance.