The Saxons were a tribe of Germanic people who originally came from the area of current northern Germany. They invaded Britain during the time of Roman occupation. Similarly the Angles came from the area of northern Germany and Denmark. The Jutes and Frisians were other tribes of Germanic people. The Jutes also invaded England but over time were either driven out of the country or merged with the Saxons and Angles.
This is how the Romans saw Britain and Ireland in A.D. 150.
At first small numbers of raiders came to Britain attacking and plundering settlements close to the shore. They then returned home to their native lands with thier stolen goods. After the Romans left Britain in 410 the country was not properly defended as the regions were ruled by different people who were too busy fighting amongst themselves to care about invaders. Over time the Saxons and Angles spent more time in Britain creating settlements in the clearings along the rivers. Eventually they moved further inland fighting for land with the Britons. The Anglo-Saxon invaders were led by many different chieftains and once Britain was conquered they created many separate states in which they ruled.
Each separate state had a king and council known as a witan which advised the king. The witan was made up from important nobles of the state. Like the Britons before them the Anglo-Saxons fourght amongst themselves. When an Anglo-Saxon king defeated the king of another state, the conquered state was either completely taken over or remained independant but had to pay a fee to remain so. During the hundreds of years of the Early Middle Ages the states were fought over many times. Several times during those many years a king came to power with the strength of conquer the majority of the other states and become the ruler of England. These overlords were known as Bretwalda.
From 616 to 632 Edwin ruled Northumbria. He was able to conquer much of northern England and combined the two Northumbrian states of Bernica and Deira.
This map shows how Britain was organised in around 550 AD
When the Saxons and Angles invaded Britain they had to defeat the native Britons. This is the era of King Arthur who, legend states, defended Britain against the invaders. The Saxons managed to defeat the Britons and take control of the country. The Saxons settled mainly in the south of England, while the Angles settled mainly on the east coast.