Note for readers outside Europe or who speak English as a second language; Celt is pronounced "Kelt".
Celts are the people who come from the Areas of Europe which are in and around Britain, these are Ireland, Scotland, Wales, Cornwall and the Isle of Man in England and Brittany in France. There are also Celts in the Basque region where France joins Spain.
The people of England were Celtic-speaking as well, and the people of the region had originally just as much claim to the term 'Celt' as any of the other aforementioned regions. Studies have shown that the people of the British Isles hardly vary at all, genetically speaking. Successive invasions by Saxons, Vikings, and Normans changed the English culture so today most English people don’t speak a Celtic language, don't have a Celtic culture and don’t consider themselves Celtic.
In Roman and pre Christian times Celts practised a particularly unpleasant religion where Human sacrifices happened regularly. This inspired a terrifying film, The Wicker Man. It’s likely that few types of Christianity cause as much suffering as the Old Celtic religion did. The Spanish Inquisition may well have been worse. The May Day festival (Beltane) and Halloween developed from traditional Celtic festivals. Today Wicca is derived from the old Celtic religion and most branches of Wicca have lost the unpleasant elements.
Many descendants of places where the Celtic culture was dominant have emigrated to other places in the world. Boston, Massachusetts in the U.S. was one city which experienced a significant influx of population from Ireland in the 1800s. At the time, there was a great deal of a reaction and prejudice against this new group -- like many others who came to the U.S. in large masses from countries where they were persecuted or didn't have many chances to escape Poverty. In Boston, some of the jobs that many of the new immigrants were willing to take and could get while integrating into their new country led to lasting stereotypes -- such as the Irish policeman. The professional basketball team in Boston is known as the Celtics, but it is pronounced "Seltics". (See the wikia on the Boston Celtics.)