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. 

The Japanese Ministry of Education announces a list of official Kanji characters which is called Joyo kanji (usual use kanji). According to the latest announcement in 2010, 2136 kanji are set as joyo kanji. This dictionary contains 14,313 individual kanji which is more than enough for to read the newspaper.
This is the complete version of Kanji dictionary which includes 50,000 individual kanji and 500,000 phrases. I like to have them just to decorate my book shelf.