What a snapshot holds
Folder totals, not files: every folder of consequence with its size and counts, compressed into a few kilobytes. Taken automatically after each complete scan, if you leave that on.
Comparing two
Pick two snapshots of the same place and DiskDuo shows what grew, what shrank and what appeared, with the change attributed to the folder that actually caused it rather than to every parent above it.
How many are kept
The thirty most recent, plus anything you pin. Pinned snapshots are never removed by that rule.