The keyboard layout governs which character is generated by pressing a key on the keyboard. You would normally install the one that corresponds to the characters actually printed on your keyboard keys. 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.