system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Tumblr. What is it? How does it work? Why is it so popular?
gpt
Tumblr is a blogging platform. It's typically best for short-form post. Think of it as an in between Twitter and a traditional blog. 

Tumblr essentially allows you to post text, photos, videos, etc. It also makes it very easy to share content between blogs. For instance if you like a particular photo you can "reblog" the post, with credit, to your blog. The entire process is pretty easy to get a hang of if you are just starting out yet still offers the ability to professionally blog easily. 

Tumblr became popular with youth and this popularity gradually led the platform to be adopted by journalists, artists, and other companies as a way to spread their content easily.