SQL JOIN:simplemente no puedo entenderlos

Si busca usando solo un término de búsqueda, su consulta podría verse como

SELECT b.*, a.*
  FROM tbl_bookdescription b JOIN tbl_authordescription a
    ON b.authorID = a.authorID
 WHERE b.bookName   LIKE '%searchterm%'
    OR b.bookReview LIKE '%searchterm%'
    OR a.authorName LIKE '%searchterm%'

Si reemplaza searchterm con 'Tom' obtendrás

| BOOKID |   BOOKNAME | BOOKREVIEW | AUTHORID | AUTHORNAME |
------------------------------------------------------------
|      1 | SQL Basics |       Cool |        1 |        Tom |

Ahora, si es 'Bueno', entonces

| BOOKID |    BOOKNAME | BOOKREVIEW | AUTHORID | AUTHORNAME |
-------------------------------------------------------------
|      2 |  PHP Basics |       Good |        2 |      Jerry |
|      4 | HTML Basics |  Very Good |        3 |       Phil |

Aquí está SQLFiddle demostración