Like the Model A & B, the B+ requires a FLASH card to store the operating system and files. The Model A and B had an SD socket on the bottom:

The SD card stuck out the end of the Pi, and could come loose or snap off by accident with enough force.

The new B+ replaces the large SD socket with a new MicroSD socket

The good news for any Adafruit customers, we always had dual Micro/Standard SD cards so you can still use the same cards! Just discard the adapter.