Every nation, country or colony often has rules and laws that control and regulate people who come in from other places. Migration becomes illegal if people do not have the permission of the country or borders they are entering into.
In recent time, illegal migration has been on the rise. Illegal migration is often fueled by pull factors.
People sneak into other countries by land or sea, and other organized groups help people to sneak into other countries to work illegally. This is known as human trafficking.
Many of the illegal migrants involved end up in very difficult and dangerous situations, as they do not have the proper documents to get a job.
The flow of illegal migrants is often from poorer countries to richer countries. The people involved often are not the poorest in their home countries. They tend to be people with a lot of information, knowledge, ambition and motivation, which often fuels their desire to migrate for better life.
These are people who live in a place without permission and the authorities have no record of them. It also includes people who visit a country for tourism or education or health purposes legally, but do NOT go back. This means even though they went there legally, they are now illegal immigrants because they have out-stayed the time period they were given. In a similar way, immigrants who have expired documents, or who came in with fake documents all fall under undocumented immigrants.