Settlers are people who are attracted to an area with the idea of living there. Usually there has to be something pushing people away from where they live. Only a few people want to explore new lands.
To be able to settle, either the land has to be empty, or they have to push the existing people out of the way, or share unused parts of the land with them.
In Europe. early settlers include the Anglo-Saxons, who lived during Roman times. Their land was simply not good enough to support them as their numbers grew. So they looked for new lands and chose, among other places, Britain. They moved in as soon as the Romans left. The main Viking settlers who went to Britain some hundreds of years later were called raiders, but their main aim was to take over land and settle. They mostly came from Denmark, not far from where the Anglo-Saxons came from.
In Britain, the Battle of Hastings of 1066 caused the Anglo-Saxons to lose control to William the Conqueror, but he had just a few nobles with him. The Normans ruled, but they were not settlers. After the Vikings there were no more settlers who could take over the land.
By Tudor times a few people wanted to make their fortunes in the new lands of the Americas. This time people left Britain and settled along the coast of North America. But the people who went for adventure and fortune were quite small. They ended up being powerful landowners and politicians, but they did not make up the majority of the settlers. Most settlers were people who felt they could no longer live in Britain because their beliefs were not accepted, or because their old way of life working the land was no longer possible.
This kind of moving to the New World was not just happening in Britain. It was something that was happening all over Europe, so that people from many countries tried to find a new life in the Americas. The Irish felt themselves forced out by harsh treatment in their own country. The Highland Scots were pushed out by the Lowland Scots during the Highland clearances. And the same was true in other countries. That is why there are so many people of German, Swedish, Italian, Greek and so on in the Americas.
Settlers were people who moved into land and took it over. Those moving along the Oregon Trail in the mid 19th century to find unfarmed lands in the west were true settlers, but they only moved west because there was no more room in the east. That had all been settled. The Mormons were forced west because people in the east did not like their customs.
Later, there would be huge movements of people from Russia. But there was no longer any new land for them to settle. So these people were not settlers, but immigrants, who came to live in lands that were already settled. And that is what has happened since.
The early arrivals in Australia did not intend to be settlers, for they were convicts. At the end of their sentences many had no alternative but to stay and make a living in Australia. They were early settlers. Since then, most have been immigrants.