What is the difference between a USA edition of Harry Potter and a UK Edition? 

There are four main differences between the USA versions and the UK versions   ....  

1. PUBLISHER: Harry Potter books are distributed in the UK by the publishing company Bloomsbury. Harry Potter books are distributed in the USA by the publishing company Scholastic. 

2. CONTENT/WORDS/STORY-LINE:  The books, as written by J. K. Rowling, contain many British slang and references that the USA publisher, Scholastic, feel are unfamiliar to American readers. The USA/Scholastic versions, replace these British references and slang terms with words more familiar with the USA audience. The story-line is the same. The page count is different between the USA and UK editions, but is attributed more to type size and page lay-out than word count. Since the UK versions are released as written by J.K. Rowling, the UK versions are truer to J. K. Rowling's intent. 

3. COVERS: The cover art is different in USA vs UK editions.

4. UK ADULT vs UK CHILDREN'S VERSION: Each Harry Potter book in the UK is released in an "Adult" version, and a "Children's" version. The content (the story, the words) in the UK is exactly the same between the Adult vs Children's edition. ... the only difference in the UK between the Adult version vs the Children's version is the cover art.