Obsidian Notes are Markdown files, but Markdown is not a widely-standardized format. However, the creators of Obsidian wanted maximum compatibility with as many Markdown formats as possible. As a result, they have their own Markdown flavor called Obsidian Flavored Markdown.

It has the following characteristics:

• It implements the CommonMark spec
• It implements some functionality from GitHub Flavored Markdown (unsure how much exactly)
• It implements some functionality from LaTeX (unsure how much exactly)
• It adds additional syntax:
  • [[ ]] for internal links
  • ![[ ]] for embedding files
  • %% for comments
  • > [!note] for callouts
• It does not support Markdown or blank lines within HTML blocks

Source: help.obsidian.md