All people in the United States have a right to be free from unreasonable searches by the government. This means that neither the police, nor anyone acting on behalf of the government, is allowed to search you, your home, your car, or any other place where you have a right to expect privacy. However, there are exceptions under the law. These exceptions allow the police to search when they have "probable cause" for example.
A criminal lawyer must evaluate the decisions made and the steps taken by the police officers involved in a criminal case. If the police broke any of the rules, namely your Constitutional rights, then they are not allowed the use the illegally obtained evidence against you. Many people argue that the system is unfair, since someone who is actually guilty may be set free due to a "technicality." However, this sort of thinking fails to recognize just how important it is that the police can't break down your door in the middle of the night just because they feel like it. The Supreme Court of the United States has decided long ago that protecting our freedom from the power of the government is just as important as prosecuting those that are guilty of crimes.
The decisions the Supreme Court has made, in case after case throughout history, have molded our laws regarding what is considered proper, or improper, police investigation. Criminal defense lawyers must be well versed in these Constitutional laws in order to effectively defend a criminal case. It may be that your lawyer will never even have to talk about the evidence, if he can instead show that there should be no evidence to begin with. The evidence may be suppressed if it was obtained illegally, and your case could be thrown out. The Fourth, Fifth, and Sixth Amendments to the Constitution are the amendments involved in all criminal matters.
The Fourth Amendment to the Constitution
You have a right not to be searched without a legally valid reason. You are allowed under the Constitution to refuse to consent to a police search. If the police search you, even without your consent, the Constitution requires the police to be able to show a judge, in court, why they had "probable cause" to believe that you had committed, or were about to commit, a crime (or possessed evidence, etc.).
The Fifth Amendment to the Constitution
You have a right to remain silent. The government cannot force you to speak. You are protected from having to incriminate yourself by answering questions. If the police force you to answer them, they cannot use your statements against you. They must tell you that you have a right to remain silent before they ask you any questions (but this only applies after they arrest you).
The Sixth Amendment to the Constitution
You have a right to be represented by an attorney. If you cannot afford one, the court must give you one for free. If the police are interrogating you, and you ask for an attorney, they are required to stop all questioning immediately. If the police continue to question you after you ask for an attorney, then all statements you made from that point on cannot be used against you later in your case.