The official answer is the Nile, which is 6,690 km or 4,157 miles long. After that the longest river is the Amazon at 6,387 km. About 5 years ago a group of scientists from Brazil claimed that they had located the source of the Amazon further south than previously thought, which if true would make the Amazon slightly longer than the Nile. However, until this claim is officially approved, you can say the Nile is the longest river (the Amazon is the widest.)

The Nile is famously associated with Egypt. In fact it flows through five countries, and its two tributaries flow through others as well, with Egypt taking only about a fifth of its course. The mouth of the Nile is in Egypt near Alexandria and it flows from there into the Mediterranean; its source, which was looked for by several explorers in the past, is in Burundi (the White Nile).

You can see a map of the Nile here.