Depending on the type of legal system, a statute may be declared unconstitutional by any court or only by special Constitutional courts which have the authority to rule on the validity of a statute.
In some countries, not having a formal written constitution, the legislature may create any law for any purpose and there is no provision for a law to be declared unconstitutional.
A constitutional violation is thus somewhat different from the breaking of a normal law, both in terms of seriousness and punishment. Declaring a law unconstitutional does not result in the punishment of those who passed it down.
In many jurisdictions the supreme court or constitutional court is the final legal arbiter that renders an opinion on whether a law or an action of a government official is constitutional.
Some examples of unconstitutional actions can be:
Most constitutions define the powers of government. Thus, national constitutions typically apply only to government actions. This means that only governments can violate the nation's constitution, but there are exceptions.
The legal encyclopedia American Jurisprudence says the following in regard to constitutionality:
The general rule is that an unconstitutional statute, though having the form and the name of law, is in reality no law, but is wholly void and ineffective for any purpose since unconstitutionality dates from the time of its enactment and not merely from the date of the decision so branding it; an unconstitutional law, in legal contemplation, is as inoperative as if it had never been passed ... An unconstitutional law is void. (16 Am. Jur. 2d, Sec. 178)