The basic idea of what things are called crimes is that they are thought to be things that might cause a problem for another person. Things like killing another person, injuring another person, or stealing from another person are crimes in most countries.
But different countries have many different ideas of what things are crimes. Some things that are crimes in one country are not crimes in other countries. Many countries get their ideas of what things are crimes from religions.
In many countries, to say that you made or wrote a book, movie, song, or Web page that you didn't really write or make is a crime against the laws of copyright. In many countries, helping to grow, make, move, or sell illegal drugs is a crime.
In most countries, police try to stop crimes and to find criminals. When the police find someone who they think might be a criminal, they usually hold the person in a jail. Then, usually, a court or a judge decides if the person really did a crime. If the court or judge decides that the person really did it, then he or she might have to pay a fine or go to prison. Some countries execute (kill) some criminals for some crimes.