SSD vs HDD
SSDs are faster and more reliable for everyday use because they have no moving parts; hard drives remain cheaper per gigabyte and still make sense for bulk storage. For a computer's main drive — the one Windows and your programs run from — an SSD is almost always the better choice today.
How they actually work, briefly
A hard drive (HDD) stores data on a spinning magnetic platter, read by a physical arm that moves across it — similar in principle to a record player. An SSD stores data electronically, in flash memory chips, with no moving parts at all. That single difference is what drives almost every practical difference between them: speed, noise, durability, and how they fail.
Speed
Because there's no physical arm needing to move to the right location on a spinning disk, an SSD can access any piece of data almost instantly. In practical terms, that means a computer starting from an SSD typically boots in seconds rather than a minute or more, programs open close to instantly rather than with a visible delay, and the computer stays responsive even with several things open at once. This is the single biggest, most noticeable upgrade most ageing computers can get — bigger, in most cases, than adding RAM or a faster processor.
Reliability and durability
Hard drives fail in fairly predictable mechanical ways over time — the moving parts wear, and physical shock (a knock, a drop) can cause immediate damage, sometimes with warning signs like clicking or grinding beforehand. SSDs have no moving parts to wear out mechanically, so they're far more resistant to knocks and shock, and tend to fail less predictably — sometimes with no warning at all, which is part of why backups matter regardless of which type of drive a computer uses. Neither type lasts forever; both are consumable components with a finite lifespan, just via different failure patterns.
Cost and capacity
Hard drives remain meaningfully cheaper per gigabyte at large capacities, which is why they're still the sensible choice for bulk storage — external backup drives, large media libraries, archive footage — where raw capacity matters more than speed. SSD prices have fallen a lot and continue to, but at very large capacities (multiple terabytes) a hard drive is still usually the more cost-effective option for storage that doesn't need to be fast.
So which one do you actually need?
For a computer's main drive — the one holding Windows, installed programs and the files you use day-to-day — an SSD is the right call for nearly everyone today; the speed difference alone is worth it. For secondary bulk storage, particularly on a desktop with room for a second drive, a hard drive still earns its place. Many desktops run both: an SSD for the operating system and everyday use, and a larger hard drive alongside it for storage that doesn't need to be fast.
Can I just add an SSD without replacing my hard drive?
On a desktop, often yes, if there's a free drive bay and connection available. On a laptop, it depends on the model — some have room for a second drive, many don't. Either way, the SSD should hold Windows and your programs to get the speed benefit; a hard drive left as secondary storage doesn't slow things down.
Do SSDs still need defragmenting like hard drives?
No — defragmenting is specifically about reducing the physical distance a hard drive's read arm has to travel, which doesn't apply to an SSD at all. Modern Windows already knows not to defrag an SSD the same way, and doing so provides no benefit.