The keyboard layout governs which character is generated by pressing a key on the keyboard. Input methods are extra pieces of software that are typically used to enter languages such as Chinese, Japanese and Korean using a Western keyboard. For example in Japanese you might enter two characters "ko" and get a menu of 200 possible Hiragana or Kanji corresponding to that syllable (こ　腰　etc) If you can generate all the characters you need directly with the keyboard then you do not need to install an input method at all.