Immigration is the action and the effect of immigrating. This verb refers to people who arrive / enter a country / region that is not the political them to install to, usually for economic reasons, or academic. In other words, these people do to have a better future (better paid employment), to flee from persecution from their homeland or to study in a school or a University's prestige.
The complementary concept of immigration is that of emigration (the output of a person from one country to another). An immigrant has necessarily been emigrante. For example: If Richard leaves the USA moved to the Chile, it is an emigrant of USA and an immigrant to the Chile.
Migration processes take place during all of human history. Usually, they are massive and coincide with moments of crisis of a given region, where thousands of people seek to emigrate to a foreign country looking for a better quality of life. Today, globalization is a social phenomenon that facilitates immigration towards the development of means of transport and the liberalization of borders.
However, immigration tends to be criticized by some social sectors of the host country. Some people find that it is because the immigrants that economic and social problems exist, even if statistics show that immigrants bring, through their labour and their taxes, more resources than what they consume. However, this does not prevent that many immigrants are subject to discrimination.