In a valid contract each person is legally bound to do what is promised. If one party to a contract does not carry out the promise, the other party can go to court and be entitled to a remedy.
First, the court must decide if a contract has been made. The most important principle guiding a judge is whether a reasonable observer, an average citizen, would decide that it was a contract. The judge will also consider if the contract has all the essential elements: an offer, an acceptance and a valuable consideration.
It is very important for a judge to consider the capacity of contractors, that is whether they are legally competent to make a contract. In English law there are special rules if one of the contractors is a company, rather than an individual, a person under 18, or an insane person.
There are special rules of interpreting contracts in which one or more contractors made a mistake or were pressured or tricked into making an agreement. There are also rules for dealing with illegal contracts. For example, an agreement to sell illegal drugs could never be enforced in court.
In common law countries, the judge will be guided by decisions made in previous cases when deciding about contracts. If the judge is dealing with a problem that has never arisen before, he must make a decision based upon general legal principles, and the decision will become a precedent for other judges in similar cases in the future.

validity                            действительность, законность
party to a contract                сторона в контракте
to carry out smth.                   выполнять ч-л.
to be entitled to a remedy         иметь право на средство судебной защиты
reasonable observer                лагоразумный наблюдатель
average citizen                                    среднестатистический гражданин
essential elements                                основные составляющие
capacity of contractors              дееспособность сторон в контракте
to be legally competent to do smth.   (иметь право по закону сделать ч-л.
insane                                                 душевнобольной  
to trick smb. into making a contract        заставить к-л. заключить контракт обманным путем
to pressure smb. into making a contract   заставить к-л заключить контракт
drugs                                                  наркотики, лекарства
to be enforced in court             (обеспечивать исполнение в судебном порядке
to arise / arose, arisen /                          возникать