Simply put, they are invaders who flout the law. They are criminals who are here illegally. There are many ways to enter legally. Student visa, vacation visa, work visa, application for citizenship, marriage to a citizen, agreement to serve in the military, by investment, seeking asylum, family based immigration and etc.
But why bother with the law? Just invade another nation. But if you do this in their country you will be jailed for a long time.
