Japanese language is written using three alphabets: Hiragana, Katakana and Kanji. However since Japan began being “connected” to the Western world there have been several attempts to write the Japanese language using our alphabet. The basic idea of all these attempts consists on emulating the pronunciation of Japanese characters the best possible way using the Latin alphabet. Even though none of the attempts has been perfect the most used method nowadays is the Hepburn romanization.
I have never taken really seriously to learn perfectly the rules to write Japanese with our alphabet because I find it quite absurd; it’s like if I would try to write English with Japanese characters. However it is true that for some people it’s quite useful to start learning the language; although I don’t recommend it. It is hard, but it is better to start learning Japanese directly with hiragana.
Notice for example this sign where the word 井の頭 is written as inoKashira and then as inoGashira. To say the truth, the word is quite complicated, technically it would be written inoKashira, but when pronouncing it in Japanese quickly it sounds more like what would be written inoGashira. It’s not something very relevant but it’s funny to see the word written in different ways in the same sign!