An asylum seeker is a person seeking sanctuary or refuge from persecution, generally at the hands of a government or its representatives. Asylum itself means sanctuary or protection. In modern times, an asylum seeker generally is someone fleeing from one country to another to avoid racial, religious, political or even sexual persecution. Historically, asylum seekers were more commonly accused criminals who would seek sanctuary from law enforcement in churches and their environs.
The concept of asylum has a long history and was practiced in ancient Egypt, Greece and ancient Israel. Part of it has to do with the sovereignty of the nation to which the asylum seeker has fled; by taking jurisdiction over the asylum seeker, the nation is asserting its sovereignty. Conversely, the nation that automatically repatriates asylum seekers can be considered to be acknowledging that the other nation's claim is superior to its own sovereignty. The concept was refined in medieval Europe, when churches were permitted under the common law to offer sanctuary to fugitives. Churches generally required authority from the sovereign to offer sanctuary, and some were permitted to offer sanctuary only within their walls, while others could offer sanctuary over a wider geographic area.
Those who fled to the sanctuary of a church, though, weren't given absolute protection. Instead, they generally earned themselves some time, perhaps a few weeks, during which they'd surrender any arms to the church and place themselves under the church's jurisdiction. At the end of this time, they'd make one of two choices: they could confess their guilt, give up all their property variously to the church and the state and go into exile, or they could proclaim their innocence and stand trial.
In modern times, the kind of sanctuary that churches can offer is very limited, and in many nations is recognized more as a courtesy than as an absolute right. When fugitives seek sanctuary in a church today, the usual outcome is the negotiation of the fugitive's surrender, at which point law enforcement is empowered to enter the church and seize the fugitive. An exception is the so-called "sanctuary movement" in the United States, where some churches and municipalities will harbor fugitives accused of illegal entry into the United States and won't facilitate their surrender to federal authorities.
When people flee official persecution in a nation, they'll generally seek out a nation that can reasonably be expected to offer them asylum based on their specific circumstances, often one of the Western democracies. Most nations have a formal application procedure for asylum seekers to follow upon their arrival, and cases are decided on a case-by-case basis. This process can be very politically sensitive, and there's no guarantee that an asylum seeker will in fact be granted asylum. However, in the latter half of the 20th century, multinational treaties have been established that establish some level of uniformity for the asylum process. In addition, some nations have started to include other reasons for granting asylum, such as sexual persecution and abuse.
The standards of asylum for criminals, on the other hand, have changed over the years. Most nations participate in extradition treaties that provide for the repatriation of criminal fugitives; however, many nations apply additional standards. For instance, some nations will only extradite fugitives for crimes they themselves recognize; that is, if a nation doesn't recognize an act as a crime, it will refuse to extradite persons within its jurisdiction to account for that crime to another nation. This can be a significant factor because some nations prosecute individuals for political and religious crimes, branding as criminals those who commit acts that in other nations are not crimes, like apostasy, fornication, and political dissent. Likewise, if the crime is recognized by both nations as a crime but the nation seeking extradition imposes a harsher punishment than would be imposed by the asylum nation, extradition might be refused.
There's sometimes confusion between asylum seekers and refugees. Refugees are large groups people driven from a region or nation for any of a wide variety of reasons, including war or other domestic unrest, natural disasters like earthquakes, hurricanes, or tsunamis, and even economic circumstances. Unlike asylum seekers, who are fleeing persecution and whose cases are decided individually, refugees are handled as a group, and individual applicants must only verify that they meet the qualifications to be included in the group.