A digital garden is where you post ideas and thoughts that are raw, in contrast to a traditional blog post that may be refined, long, and well supported.
Digital garden posts (seedlings) are for you to plant, cultivate, and explore. They encourage discussion, and aren't meant to be scrutinized like you might do with a traditional blog post.
Seedlings are not marketing focused.
For me, the digital garden concept is very liberating. I've often felt overwhelmed by writing traditional blogs, because of the weight of it having to be something.
With a digital garden, I feel encouraged to write without fear or anxiety around how my raw ideas will be perceived.
Resources
Maggie Appleton has a great garden with a visual queue of the progress of her seedlings.
Joel Hooks shares his thoughts on what a digital garden is for him.
This is a great Egghead discussion around digital gardens. The discussion covers MDX as well, but the digital garden topic comes in at 28:56.
Chris Biscardi has a great explanaition as well, with nice list of all of his seedlings.
Josh Branchaud gives a great outline of digital gardens, with a lot of resources.