What is the difference between 2-HD and 2-DD floppy?

There have been three formats of 3 1/2" Floppy disk: - 3½-inch HP (single sided) - from 1982 - 280KB - 264KB Formatted Storage Capacity - 3½-inch DD (double sided) - from 1984 - 1 MB - 720KB Formatted Storage Capacity - 3½-inch HD (double sided) - from 1987

• 1.44 MB - 1440KB Formatted Storage Capacity