Durable block storage for Compute Engine VMs. Persistent Disk can be zonal or regional (with synchronous replication between two zones); Hyperdisk lets you provision IOPS and throughput independently of capacity. Local SSD, by contrast, is ephemeral.
On the ACE you pick a regional disk (or Hyperdisk Balanced High Availability) when the disk must survive a zone failure, and Hyperdisk when you need to tune performance without enlarging the disk.
Persistent Disk and Hyperdisk is a concept that can show up on the exam. Check the full guide to see it in context.