Hello! I understood this way:
Kanji are characters that have an idea of their own and are unique in China. The Japanese use several of these characters, and have two readings of the same: reading Chinese and Japanese reading. The same kanji can be read two ways, or more, in different words.
The hiragana is a simplification of Kanji made by women so they could communicate and write their own stories.
Katakana is also a simplification of kanji, but was created by Buddhist monks.
The Romaji is how to write a Japanese word in Western characters.
I hope have helped a bit.