The name 'Viking' comes from a language called 'Old Norse' and means 'a pirate raid'. People who went off raiding in longships were said to be going 'a-Viking'.
Britain was a good place to raid because its monasteries had many treasures in them to steal, such as gold coins and jewels. The Vikings weren't Christians and because the monks living in the monasteries had no weapons, they were easy targets.
The Vikings first invaded Britain in AD 793 when they attacked the monastery on Lindisfarne, a holy island on the north-east coast. A few years later they looted the the island of Iona (off the west coast of Scotland) and killed its monks.
The English called the Viking invaders 'Danes' even thought they came from other Scandinavian countries too.
Viking armies usually had between 1,000 and 2,000 men in them. They fought using long swords and axes. For protection, they had a round, wooden shield and wore helmets made of leather or iron.