The Second Age is a time period from J. R. R. Tolkien's Middle-earth fantasy writings. Tolkien intended for the history of Middle-earth to be considered fictionally as a precursor to the history of the real Earth.
This age was characterized by the rise of Númenor, the rise of Sauron in Middle-earth (which saw the creation of the Ringwraiths), and the early wars of the Rings between Sauron and the Elves.
The 3441 years of the Second Age are, for the most part, unchronicled, unlike the First Age which is largely recounted in The Silmarillion, and the Third Age, which is the time period during which The Lord of the Rings is set. Besides the hints about the Second Age found in The Lord of the Rings, several shorter writings fill in some of the gap between the downfall of Morgoth at the end of the First Age and the first downfall of Sauron which marked the beginning of the Third Age.
"The Tale of Years" in Appendix B of The Lord of the Rings outlines the major events of the Second Age, especially as they relate to the Rings of Power and the events and characters of The Lord of the Rings. Appendix A of The Lord of the Rings contains genealogies of the royal house of Númenor, and several sections of Unfinished Tales of Númenor and Middle-earth deal extensively with Númenor and several of its kings. Also, at the end of The Silmarillion, "Akallabêth", or 'the falling of a star,' recounts the fall of Númenor and its kings, and also the rise of Gondor and Arnor. There are several other writings dealing with the Second Age, but because of the many stories only hinted at in the extant sources, the Second Age remains vague.