Intro to Katakana
Katakana is used for foreign words, such as ピアノ, (pi:a:no). The characters in Hiragana and Katakana all represent sounds, while all of the Kanji characters represent things.
Like Hiragana, the Katakana alphabet also has 46 basic characters. As you look over the Katakana chart below, you will notice that some characters are very similar to their Hiragana counterparts, while some look very different.
Just as in Hiragana, there are also characters that are similar to the ones above, but are altered with two small dashes or a small circle on the top-right of the characters. For example ヒ (hi) with two dashes would be ビ (bi) but with a circle would be ピ (pi). And aside from all of these, there are, again, also characters that are smaller versions of normal characters that alter the way the character before or after them sounds. For example, ファ (fa). フ+ァ is fu+a, but the a makes the u obsolete. For a complete chart of Katakana characters, and for more information on Katakana, you can visit the Wikipedia article on Katakana.
To practice and memorize katakana, you should try out the katakana quiz found on this website.