Do I have an HDD or an SSD in my MacBook?
All retina Macbook Pros have PCIe SSDs. Previous MBPs (2012 + earlier) shipped with HDDs.

To find this out, find your model information by going to Apple (top left corner of the screen) -> About this Mac. Find the year (Mid 2014), then use either Apple's website or a website such as everymac.com to determine the specifications of the storage originally shipped with the computer.