A century consists of exactly 100 years. Because this is such a long time and a "round" number, it is easy to use it to divide history. Our calendar begins with the birth of Jesus Christ in the year 1, even though Jesus was probably born earlier. The first century then lasted from the year 1 to the year 100. We ourselves now live in the 21st century.
When we began to reckon time, zero was not yet known as a number. That is why the first year is the year 1. The 21st century therefore began on 1 January 2001, even though many people in the world celebrated it on the night of 1 January 2000.
When one speaks of the "so and so" century, one must be careful: The year 1950 is not in the 19th century, despite the 19 at the beginning, but in the 20th century. You can imagine it like this: A child who is only a few days old is already in the 1st year of life. This only ends with the 1st birthday, just as the 21st century ends with the year 2100.
Often, when we think of a century, we think of certain inventions and discoveries from that time. For example, the 19th century is called the century of the railways and the 20th century the century in which the car became established.
A millennium comprises 10 centuries. The millennium is also called the millennium. This comes from Latin. The 1st millennium lasted from the year 1 to the year 1000 AD. So today we are already living in the 3rd millennium. There are also millennia in the time before Christ. This concept of time is often used when talking about periods in history that already lie far back in time.
One also speaks of centuries before Christ. The first one lasted from the year 100 BC to the year 1 BC.