An HDD (hard disk drive) stores data on spinning magnetic platters read by a mechanical arm, while an SSD (solid-state drive) stores data on flash memory chips with no moving parts. That design difference is why SSDs are much faster, quieter, and more shock-resistant, and why HDDs still win on cost per terabyte for bulk or archival storage.
According to IBM, SSDs commonly reach far higher transfer rates than HDDs (often thousands of MB/s versus hundreds), use less power, and suit everyday boot drives and apps; HDDs remain useful for large, less frequently accessed backups and archives.
Source: Hard Disk Drive (HDD) vs. Solid State Drive (SSD) — IBM