Ezra continues the history of 2 Chronicles, as you can see from the fact that the beginning of the book is the same as the end of 2 Chronicles. Ezra and Nehemiah were originally one book, and they tell the story of what happened to the exiles when they came back to the land of Israel after 70 years or more living in exile in Babylon.
It is important to note that there are three separate returns from Babylon to Israel. The first is described in Ezra 1-6 and it is led by Zerubbabel the governor and Joshua the high priest. The second takes place about 60 years later and is led by Ezra himself (chapters 7-10).
Notice the problems that the returning Jews face. First, they have external opposition from those who live around them and don’t want them to build Jerusalem or the temple (especially chapter 4). Second, they have internal difficulties because some of the men are marrying foreign wives. This is the same sin that Solomon was guilty of that caused him to commit idolatry. Ezra tears his clothes because he fears that this intermarriage can eventually cause the Israelites to become like “Canaanites.” This would be a great irony, for though the people were preserved from assimilation outside of Israel, they may become assimilated inside the land of Israel. If they are assimilated, that means there is no more Israel, no more people of God, and no fulfillment to God’s promises, including a Messiah who will save the people from their sins.