Copyright infringement is an extremely complex part of the law that can span both civil and criminal court systems. While students at all levels of the school system are taught that plagiarism is wrong -- and most adults probably realize you can't just use someone else's ideas without crediting them -- it's possible to infringe on someone's copyright without criminal intent.
Part of the reason it's possible to infringe on copyright without realizing it is that there are so many different shades of use. Under fair use laws, certain bodies of work and ideas are up for grabs if you are teaching, reporting in a journalistic function, or conducting professional reviews and criticism. But since not every work is available under fair use -- and not every activity allows a fair use approach -- you might stumble into gray areas without realizing it.
Some artists and creators also put their work into the public domain or provide a limited license for use. Licenses let you use a work if you credit it appropriately, and each license has different rules. Some works are licensed for full commercial use; some are not licensed for commercial use at all. Some are licensed for use without alteration; some works you can alter as long as you credit the original creator.
These are just a tiny tip of the copyright law iceberg, and you can see how complex it's already getting. This is why if you are accused of criminal copyright infringement, then you need a strong defense and a lawyer that understands exactly how copyright law works. Relying on your own understanding likely landed you in trouble in the first place, so don't hesitate to seek professional assistance.
Source: Reporters Committee for Freedom of the Press, "How to avoid copyright infringement -- Legal action to protect a copyright," accessed April 22, 2016