Let’s get started from the question that became the title of this post. I tried to find out the answers from variety of sources. Let’s discuss first about virus.
Because those crack, keygen, and patch work almost similar with viruses, they differ in the usage only.
A computer virus is a computer program that can multiple or self copy and spreading by inserting its own copies into another program or documents. This virus works similar as biological virus, which spreading through inserting into living cells. Computer viruses is destructive (e.g. by destroying the data on the document), disturb users. Surprisingly there are viruses that cause any effect at all.
Initially, viruses were created to test how tough of a program generated. However, as time goes by, the viruses were often misused and designed to harm.
The viruses work depends on their purposes. We are not going to elaborate them here.