Difference between JOIN and INNER JOIN

They are functionally equivalent, but INNER JOIN can be a bit clearer to read, especially if the query has other join types (i.e. LEFT or RIGHT or CROSS) included in it.

No, there is no difference, pure syntactic sugar.

INNER JOIN = JOIN

INNER JOIN is the default if you don't specify the type when you use the word JOIN.

You can also use LEFT OUTER JOIN or RIGHT OUTER JOIN, in which case the word OUTER is optional, or you can specify CROSS JOIN.

OR

For an inner join, the syntax is:

SELECT ...
FROM TableA
[INNER] JOIN TableB

(in other words, the "INNER" keyword is optional - results are the same with or without it)