Short answers, pulled from the story.
By convention, the Western Roman Empire ended on the 4th of September 476, when the Germanic leader Odoacer deposed the emperor Romulus Augustus in Ravenna. However, the legal western emperor Julius Nepos continued to rule in exile until his murder in 480, and some historians mark that date, when the eastern emperor Zeno abolished the western court entirely, as the true end.
The empire's vast size made central governance from Rome impractical, and communications by ship or mounted postal service were slow. Emperor Diocletian institutionalized the east-west division in 286 by appointing a co-emperor to govern the western provinces, a system expanded into the four-emperor Tetrarchy in 293. After the death of Theodosius I in 395 the empire split permanently between his two sons.
The last emperor actually resident in Italy was Romulus Augustus, deposed by Odoacer on the 4th of September 476. However, Julius Nepos, who had been forced out of Italy in 475, was recognized as the legitimate western emperor by Constantinople and continued to rule in exile in Dalmatia until his assassination in 480.
The collapse resulted from a combination of factors: repeated civil wars and usurpations dating to the Crisis of the Third Century, barbarian invasions across the Rhine and Danube, the loss of the wealthy North African provinces to the Vandals in 439, economic decline, and the inability of the western court to fund a professional army. The wealthy senatorial elite refused to approve sufficient funding for defense while also barring their tenants from military service.
Odoacer was the leader of the Germanic foederati in Italy. After deposing Romulus Augustus on the 4th of September 476, he proclaimed himself ruler of Italy and negotiated with the eastern emperor Zeno, who eventually granted him patrician status as viceroy. Odoacer minted coins in the name of Julius Nepos, maintained the Roman Senate, and governed using Roman administrative systems until Theoderic the Great of the Ostrogoths defeated him in 489 and killed him in 493.
Latin evolved into the modern Romance languages, including Italian, French, Spanish, Portuguese, and Romanian, spoken today by more than 900 million native speakers worldwide. Roman law, especially the Corpus Juris Civilis collected under Justinian I, forms the basis of civil law, which operates in some form in about 150 countries. The Latin alphabet became the foundation for more writing systems than any other script in the world.