As Chidoriashi pointed out, Katakana was made by taking a part of kanji.
There used to be no writing system in old time Japan, then kanji was "imported" from China.
Each kanji was used just to represent each mora, that means one had to write down all these complicated kanji to write even for just a simple word. That was too troublesome when students (priests at that time) to take notes, and they simplified kanji by taking a part of it - that's how katakana was "created".

Here's the chart to show how each kanji became katakana.
http://en.wikipedia.org/wiki/Katakana#History