What is difference between JOIN and simple query
There is no difference: your two examples are completely equivalent but using different versions of SQL syntax. The database engine will handle them in exactly the same way.

Your first example is using an explicit join and is the preferred syntax these days. It was introduced in the SQL-92 standard and is supported by pretty much every SQL-style query engine.

Your second example is usually referred to as an implicit join. It is an older syntax but still supported by most SQL based query engines and is still in the standard (so is considered to be correct even though the newer syntax is preferred). It quickly fell out of favour with the introduction and wide support of explicit joins as these are usually clearer and easier to read &amp; maintain (though the older syntax could be said to have the advantage of being more concise).

See https://en.wikipedia.org/wiki/Join_(SQL) for further examples of this and other join types, including some engine specific variations.