Do I have an HDD or an SSD in my MacBook? I have a MacBook Pro (Retina, 15-inch, Mid 2014). I am running macOS Sierra
10.12.4 . Is there any way to find out if I have a hard disk or a solid state disk? (I have a total of 250 GB of storage if that is necessary)
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.