Let's begin with some basic terminology.
Many names we use in the course are deceptively familiar. Just as it is difficult to understand British English without practice (British "subway" = our "underpass"), British geography and ethnography -- the names of the peoples who live or have lived there -- takes some getting use to.
Britain, the Britons
The Romans called the biggest of the two islands NW of Gaul -- the country we know as France -- Britannia. The people who lived in the Roman ruled part of the island were called Britons (by the Romans!). This was a general name, because the Britons were divided up into a number of peoples and tribes. Confusion is possible because the name of Britain is applied to the modern United Kingdom of Great Britain and Northern Ireland, and the citizens of that country are often called Britons.
Scotland, the Scots
To the north of Roman-ruled Britain was a country called Caledonia, roughly equivalent to modern Scotland. At this time there were no Scots in Scotland. They still lived in Ireland (then called Scotia or Hibernia). Caledonia was inhabited by the Picts. During and after the fifth century A.D., a number of Scots crossed to the west coast of Caledonia and settled there. After a number of centuries the King of the Scots became King of the Picts as well. By about 1000, the land ruled by the king of the Scots (Caledonia) became known as Scotland (Scotia), and later, all those who lived there, whatever their ancestry, became known as Scots.
What about the English? The English of the early middle ages were a group quite distinct from the ancient Britons. After the Roman withdrawal from Britain in the fifth century, migrants from the Continen set up their own kingdoms in Britain and conquered much of the south and east of the island. These people called themselves various names: Angles, Saxons, and Jutes; they may have included Frisians and Franks. They spoke similar languages, which were related to modern German. By the year 880 these languages were being considered a single language called English. There was a single king of the English people soon afterwards. The English were sometimes called Saxons, or by learned men Anglo-Saxons (to distinguish them from the Saxons still living in Northern Germany). Modern scholars often call the English who lived before 1066 Anglo-Saxons. I will usually call them the English, and their language English, because that's what they usually called themselves. The English eventually conquered most but not all of what had been Roman Britain. That area became known as England.
What happened to the ancient Britons?
Beginning in the fifth century, some went to a part of Gaul called Armorica. There they became the dominant culture, and that area has long been known as Brittany. The name Great Britain (in French, Grand Bretagne), meaning the island of Britain, was invented to distinguish it from little Britain (Brittany, Fr. Bretagne), where some of the descendants of the ancient Britons still live.
Other Britons stayed home. The majority were conquered and absorbed by the English. In the western part of the island, some Britons remained independent of the English. They were concentrated in three areas: Cornwall, Strathclyde, and the area we call Wales. The last name comes from the term the English used for the Britons, Welsh, which means foreigners. The Welsh called themselves Cymri. Eventually Cornwall was absorbed into England, and Strathclyde was split between England and Scotland. Wales was conquered too, but has retained a separate identity.
In ancient times there was a people, or group of peoples, that the Greeks called "Keltoi." In modern times the linguistic term Celtic, has been attached to an entire branch of the Indo-European language family, which includes the ancient British, Gallic, Irish, Scottish, Breton, and Cornish languages. It is currently fashionable to think that you or your nation descended from the Celts of old, just as it was once the thing to be a remnant of the Ten Lost Tribes of Israel. The problem with using the term Celts in regard to the Middle Ages is that no one at that time considered himself to be a Celt. So I will avoid the term.
Why study medieval England instead of medieval Scotland, or Wales, or Britain as a whole?
England and the English, or at least their rulers, have been the dominant power in the British isles for a long time. We recognize this every time we use the phrase "Queen of England." There is no title "Queen of England" today -- Elizabeth is Queen of the United Kingdom of Great Britain and Northern Ireland. But England has absorbed the other cultures of the British islands, and the mix of institutions, language, and ideas we call British has a predominant English element. If we want to understand the British heritage in the English-speaking countries of the world, we must understand England first.
Another reason to study medieval England: it gives us an opportunity to study some of the developments of the European Middle Ages within the limits of a single country. England is not a "typical" European country -- which one is? -- but is an important one. If you understand how the Middle Ages affected it, you have a good start to understanding the Middle Ages in Europe as a whole.