We pick the leaves of coca, we stack the crates of cola
We wait upon the tables where you dine
And learn from you not to accept the little that we're given
To take the piece of silver where we can
Now in the queues at immigration, in the border zone
We are your bastard children, all coming home
- New Model Army
An illegal immigrant is a person who moves to a different country on a long-term basis without the necessary approval from the host country's government.
Illegal immigrants are in some ways the lowest of the low in the modern world. Not only do they start over, usually with limited resources; they have a difficult, sometimes expensive and dangerous task getting to their destination; and then have to find employment that they are not legally entitled to. They are disadvantaged over the other people around them with regards to healthcare, housing and other state assistance. Then they have to deal with ethnic prejudice, xenophobia, and prejudice against their illegal status: "come over here and take our jobs", "can't speak the language", "sponge off welfare", etc. They may live in fear of arrest and deportation or incarceration. Many countries treat illegal immigrants in ways that they would not treat their own citizen law-breakers, e.g. subjecting them to prolonged detention without trial if arrested.
Why would anyone become an illegal immigrant? Because it's better than the alternatives. First world crumbs can work out to more than the whole third world plate.
Some illegal immigrants, as with any group of people, must be crooked or greedy. Many aren't.
Politicians, especially right-of-centre ones, often use illegal immigrants as a bogeyman. They don't vote. They break the law by existing, and so are by definition responsible for crimes. They need money and often have no employment, and so are tempted by theft. I'm sure that even when not wishing to harm others, they have great temptation to break the rules. E.g. forge a document in order to get a bank account. Many illegal immigrants will come into contact with career criminals, e.g. gang bosses who organise illegal work-gangs. These mafias often form along ethnic lines. For a person not au fait with the local language and bureaucracy, these social structures may be a lifeline.
For an illegal immigrant to find work, they must look to the informal economy. Here there is an implicit social contract: The Employer agrees to overlook the illegality of employing the immigrant, and the immigrant agrees to overlook the sub-minimum wage, and/or other violations of minimum working conditions such as safety, health risks and hours worked per week. It follows then, that in several parts of the world, entire parts of the economy, particularly casual or manual labour, depend heavily on illegal immigrants 1.
Illegal immigrants are making a better life for themselves, but do they benefit the host country? The host country has already implicitly made a decision that they do not wish the person to reside there, by having immigration criteria that this person does not meet. Young, rich, healthy doctors seldom have trouble applying for citizenship (though the height of the bar may vary depending on where you come from. It may be harder to get your qualification accredited if you come from a poor, insignificant country). The immigration criteria are however usually a political football, and may be based on nationalistic criteria.
Are illegal immigrants making better lives for themselves to the detriment of others? Maybe sometimes, but the large-scale use of illegal immigrants in labour forces suggests that this is not the whole picture. Their employers benefit greatly from cheap, easily-exploited labour, and this benefit knocks on, ultimately to many consumers who enjoy cheaper produce. There is thus vested interest in the status quo being maintained, and in these workers being neither legalised or expelled.
Q: What's the difference between an immigrant and a refugee?
See also illegal alien, Asylum seeker, refugee and economic migrant.
1) According to wikipedia, it is estimated that over half of crop workers in the USA are illegal.