Charlemagne, or Charles the Great, was the King of the Franks and is called the "father of Europe" by many historians. Here I will discuss some of his accomplishments.
First, I'd like to talk about his greatest impact on European history by founding the Carolingian Empire. Charlemagne united much of Western Europe under one Empire through conquest. Charlemagne came to rule territory in the modern-day countries of Italy, Germany, and Spain, among others. Prior to his conquest, rule in these areas was primarily held by local kings who warred with each other often.
Directly related to his expanded Carolingian Empire was the re-unification of the secular and religious powers in society. Though Charlemagne had already acquired much of Western Europe, he was not coronated by the Pope until the year 800. This partnership between the Catholic church and the Emperor Charlemagne created the Holy Roman Empire, where the secular and religious powers served to back each other up in their authority.
Third, Charlemagne is known to have had a high regard for education, and initiated a number of education reforms which inspired the Carolingian Renaissance. Charlemagne was quite studious and encouraged his students and court-members to also study rhetoric, logic, arithmetic, and astronomy. Unfortunately, he was not very good with reading or writing, but many of the scholars he supported created and preserved beautiful Latin manuscripts.
Fourth, one of the results of the Carolingian Renaissance which still impacts us today was the creation of the Caroline Miniscule script. The purpose of this style of writing and printing was to implement a common script to be used throughout the Empire. Before the use of the Caroline script, texts (primarily religious) varied greatly in language and script dependent upon where they were produced. With the implementation of the Caroline miniscule, the very same text could be read by anyone throughout the Empire.
With the fifth and last accomplishment I will detail, I do not intend to lend any anti-Islamic sentiment to my answer. Charlemagne was responsible for the Frankish conquest of some territory which is in modern-day Spain. Prior to his efforts, Moorish Spain had been part of the Muslim Umayyad Empire for many centuries. Charlemagne and his Frankish forces successfully recaptured Hispania as territory of the Frankish-Carolingian Empire. Again, I do not mean to imply any Islamophobic sentiment, only that Charlemagne succeeded in capturing a territory which had resisted re-integration to (what we now know as) Europe for some time.