The longest river in the world is generally considered to be the Nile River, with a length of about 6,650 to 6,700 kilometers. However, there is some debate because the Amazon River, if measured including certain estuaries, can be considered slightly longer at approximately 6,750 kilometers.

Key facts about the Nile River:

• Length: About 6,650 to 6,700 km
• Flows through 11 countries in Africa
• Source: Lake Victoria (with the farthest source in Burundi)
• Empties into the Mediterranean Sea
• Supports agriculture, fishing, and has significant historical and economic importance

Key facts about the Amazon River:

• Length: Approximately 6,400 km to 6,750 km (if counting the Par estuary)
• Flows primarily through Brazil and Peru in South America
• Has the largest discharge volume of any river
• Source discovered in 1971 in the Andes of southern Peru
• Has multiple mouths and estuaries, making exact length measurement challenging

The difference in length mainly depends on measurement criteria and definitions of the source and mouth of these rivers, which is why the title of "longest river" can vary based on different studies and perspectives.