The HDD (Hard Disk Drive) and the SSD (Solid State Drive) are two of the most common options today for storing and retrieving data. These are your "disks".
Hard Disk Drives are what a lot of people may have heard others call the "hard drive". This phrase is accurate for HDDs and SSDs - they're both "hard" (physical) drives inside of and connected to the system, and they're used for storing a lot of information.
Here's what an HDD looks like:
An SSD is more compact in its design and a lot "cleaner" looking:
They're much smaller as you can see below where I've included my car key fob and HDD for scale:
And there is a second form factor that an SSD comes in, called an M.2 based on a technology called NVMe:
Both the HDD and the SSD are often referred to as "drives" or "block devices". The term "drive" (probably) comes from the fact we call them drives in Windows, like "C: drive" or "Z: drive".
The term "block device" is more complicated, but in short it means a device on which a file system stores "blocks" of information. This is a complex topic and one we'll cover in a bit more detail when we look at file systems. For now, just keep the terms "drive" and "block device" in mind.