HTMLCollection is a collection of HTML elements taken from the document. All the adjustments in DOM are automatically applied to HTMLCollection. The getElementsByTagName() method returns an HTMLCollection.
Nodelist is a collection of nodes. A method querySelectorAll() returns a collection of nodes.
HTMLCollection and Nodelist are very similar. They are both a collection of elements taken from a document. They both have a length property. The difference between them is that Nodelists can only be accessed with their index number while HTMLCollection can be accessed with a name, ID or also an index number. Another difference is that HTMLCollection is always live which means when you add an element to the page HTMLCollection will also change. A Nodelist is static which means adding element doesnt change it.
In my summary I want to state an HTMLCollection and a Nodelist are a pretty similar thing with a couple differences. They are both a part of DOM and both consist of collections of elements from the web pages.