The Celts were one of the many groups of people that lived in parts of Western Europe during the Iron Age.
As Romans expanded into the continent they entered in contact with this group of "barbarians", and eventually came to dominate them.
There were other groups, such as for example the Germanic tribes, that in time would be partially responsible for the fall of the Roman Empire.
This is a link to the Time Compass episode that deals with the Celts. It is only about 11 minutes, so go ahead and watch it. It will give you a better idea of what other peoples occupied the continent, besides the Romans.
This other interesting link will take you to a BBC site with animations and interactive activities/games. There you can learn about the life and culture of the Celts in Ancient Wales: from the mundane to the mystical. It is kind of goofy but fun.
As stated above, the Romans entered in battle with the Celts (and other peoples of Europe) as the Romans expanded and assimilated them into their Empire. The video below is a very short clay-mation rendition of what normally ensued: conflict followed by conquest.
VIDEO: Romans and Celts Battle
The video below is another depiction of a battle between the Gauls and the Roman based on the famous Franco-Belgian comic. The liquid you see the Gauls drinking is a "magic potion" brewed by the village's druid (shaman or spiritual leader) supposed to make them strong and invincible. For those of you who have read the comics once our heroes have had their "potion" the Romans stand no chance against them, no matter how organized or mighty their armies may be. Remember that this is just a comic, so no, it is not real, but it is fun to watch, all the same, just to see the contrast between the two armies :)