What is the difference between SSD and flash drives in the latest MacBook Pros?

What is the difference between an SSD and the flash drive in the latest MacBook Pros? The only difference I've noted is the price, because MacBook Pro with retina display costs $2799 with a 512 GB flash drive. At the same time, MacBook Pro without the retina display costs $2199 plus $800 for a 512 GB SSD, for a total of $2999, and a difference of $200 from the MacBook Pro with retina display.

So what is the difference between the two types of drives?

Solution 1:

Usually when Apple says "SSD" they refer to a removable/upgradeable solid-state (flash) storage device. The MBP with Retina Display (and the MacBook Airs) are also using solid-state storage, but they are non-removable.

See also: http://www.pcmag.com/article2/0,2817,2371288,00.asp