I found your question very interesting so trying to give you the required information. So first of all you must understand the term fragmentation it means that there is the unit which is divided into the parts which are called as fragments.
Internal fragmentation: Consider a Paper Book which is collection of the pages and the text is divided into pages. When the chapter's end it isn't located at a very end of a page and then the new chapter will start from the new page and there's the gap between those chapters and it is the waste of the space the chunk is also called as page for the book and it also has the unused space which is inside and called as "white space"
External fragmentation: Consider you have the paper diary and in that you did not write anything sequentially page after page but you have written it randomly. In that case you may find yourself in situation where you want to write some pages in a row but you are unable to do it as there are no required number of clean pages one after other you can find 20 clean pages in your diary totally but the pages are not adjoining.