Since Quill.js doesn't seem to document its strange jargon-y terms anywhere, here's a glossary that I've put together for it. No guarantees that it's correct! But I've done my best.
Quill - The WYSIWYG editor library
Parchment - The internal model used in Quill to implement the document tree
Scroll - A document, expressed as a tree, technically also a Blot (node) itself, specifically the root node
Blot - A node in the document tree
Block (Blot) - A block-level node
Inline (Blot) - An inline (formatting) node
Text (Blot) - A node that contains only(!) raw text contents
Break (Blot) - A node that contains nothing, used as a placeholder where there is no actual content
"a format" - A specific formatting attribute (width, height, is bold, ...)
.format(...) - The API method that is used to set a formatting attribute on some selection