The Lumia 640 is a 5-inch mid-ranger powered by Windows Phone 8.1 with Lumia Denim update. Under the hood, there is a 1.2 GHz quad-core Qualcomm Snapdragon processor, 1GB of RAM, an 8 megapixel rear Lumia Camera 5 with LED flash, 8GB of internal storage, microSD card, and a 2500 mAh battery.
XL is for Extra Large
The Lumia 640 has a bigger sibling, the Lumia 640 XL, which features a 5.7-inch display, same OS version, processor and RAM as the 640, but with a more capable 13-megapixel Lumia Camera 5 with ZEISS optics and LED flash, as well as a beefier 3,000 mAh battery. Lumia 640 XL will be available in single SIM and Smart Dual SIM variants.
Both devices come with all the standard suite of apps and services from Microsoft, and both will be upgradable to Windows 10 when that is launched.