Eurovision is a song contest made in the 1950's in Switzerland. It was more or less a political move to bring nations closer after the second world war, and the first few contests had about 5-8 entries, one from each country.
As the years went on, the song contest became popular over the station European Broadcasting Union and other channels like BBC or RTV1. In order to participate, a country has to meet a deadline to register, but before they do, many of the countries often have several weeks of "rehearsals" or mini contests between the country's top 5 or top 10 singers and which ever the audience adores the most, becomes the entry for the nation.
It's free for any country to enter as long as they are within the European border which extends as far as Turkey, with the exception of Azerbaijan, who is in Asia. The countries who rank in the top 5, are automatically signed up for the following year's contest, but the others have to register.
As far as judging goes, countries call in, or vote on the singers to award them points and the ones with the most points moves on to the semi-finals and finals. Once the winner of the final is declared, the next song contest is hosted in their country.
All in all, it's just a political way of bringing countries together and it works. Of course, over the years, there have been incidents, such as in 2009 when Georgia dropped out of the contest because it was being hosted in Russia, and at the time, Russian troops were attacking Georgia's land Ossetia.
Was used to a single program, but for various years has been necessary to have semifinals before the main spectacle. The reason is that there is so many countries that quieran to participate since not all they fit in a show.
There are two semifinals before the final spectacle. The countries to vote for the better songs of each semifinal. The 10 countries of each semifinal to come to participate in the final spectacle. There are five countries that to go directly to the end without having that to compete in the semifinals. Those are the winners of the previous year (= the host of the contest of this year), along with the following countries: United kingdom, France, Spain and Germany. Those four countries have large populations and therefore, they pay more in the European Union of diffusion, for which to obtain special privileges in the other countries.
Thus, in the end they are the 10 better songs of the first semifinal, the 10 songs of the second semifinal, the host country and the four main sponsors, a total of 25 countries.
I've only heard it used in context with a yearly song contest in Europe. It's called The Eurovision Song Contest. Each country in Europe submits a song with an artist - and it's shown on TV for a few weeks, very popular. A lot of singers have had their careers started that way. Abba sang Sweden'e entry in 1974, the song Waterloo.