Kanji (漢字) is one of the Japanese wrting system along with Hiragana and Katakana. Both Hiragana and Katakana were made from Kanji by simplifying the character. Kanji was first created in China and is an ideogram. During 5th-6th century, Kanji was imported from China to Japan by Japanese monks and students who studied there. Traditionally kanji were mostly used by men or well educated women, and Hiragana was invented for women to use.
One Kanji usually has more than one way of reading and there are more than 10,000 kanji. Obviously it's imporssible to remember all characters so that Japanese people use special Kanji dictionary.