Solid State Drive...also known as SSD. It's a phrase you might have heard of before, but might not have really understood what it meant. SSDs are little blocks of magic that dramatically decrease load times, increase read/write speeds, and generally make jaws drop.
But let's take a step back and revisit the noble hard disk drive (HDD). HDDs are physical metallic disks, spinning inside a metal enclosure, with a device that reads and writes information onto the disks (platters).
HDDs have a great size/price value, which has become even more substantial in the recent months with technological advances in platter size/amount of platters-per-drive. HDDs can be teamed up in varying arrays, to create faster speeds and/or increased reliability. A Western Digital Black 1TB HDD can generally boot Windows 10 in about 30sec-1min, which is a far cry from Windows XP days.
Back to SSDs...
Take a normal flash drive; it has no moving parts, so it is technically "solid-state". Depending on the type of flash drive (and USB generation), it can be pretty zippy. SSDs are basically amped-up flash drives. SATA SSDs are lighter than a deck of cards, can withstand high shock (no moving parts), and can boot Windows 10 in 7 seconds or less. I don't know about you, but that's faster than it takes me to go get a drink....or do anything really. And SATA SSDs are the slowest of the bunch.
Now, all that hot nasty wicked speed can come with decent storage, but it's not cheap. A 250GB Samsung 850 EVO is about $100, which is the same price as the WD 1TB Black from above. SSDs typically scale respectively in storage, so the same 850 EVO in a 120GB model is about $50, the 500GB model is about $200, and the monster 1TB model is about $400. M.2 SSDs and PCI-E SSDs are even faster, and even more expensive.
What most SSD users tend to do, is use a smaller size (120-240GB) SSD for boot and operations, and a larger 500GB-2TB HDD for bulk storage. This allows boot times and software to run fast, while saving money on big storage for files that don't need raw speed.
SATA SSDs can also be teamed up in arrays like HDDs can, which tends to break the fabric of space-time, make jaws drop, and make babies cry. This also results in a massive police chase, since the SSDs are clearly breaking the speed limit in grand fashion.
Once you go SSD....you don't go back.