Org syntax includes pre-defined blocks (see Paragraphs and Literal examples). It is also possible to create blocks containing raw code targeted at a specific back-ends (e.g., ‘#+BEGIN_LATEX’).
Any other block is a special block.
For example, ‘#+BEGIN_ABSTRACT’ and ‘#+BEGIN_VIDEO’ are special blocks. The first one is useful when exporting to LaTeX, the second one when exporting to HTML5.
Each export back-end decides if they should be exported, and how. When the block is ignored, its contents are still exported, as if the opening and closing block lines were not there. For example, when exporting a ‘#+BEGIN_TEST’ block, HTML back-end wraps its contents within a ‘<div name="test">’ tag.
Refer to back-end specific documentation for more information.