Katakana is one of four writing systems used in Japanese (the others being hiragana, kanji, and rōmaji).
Katakana is used primarily to represent foreign names and loan words that have been incorporated into the language.
Each of these katakana has an identically-pronounced hiragana equivalent.
Also, there are about 30 extended katakana that represent sounds that don’t exist in native Japanese words, such as ティ (‘ti’) in パーティー (‘party’).
As such, extended katakana have no hiragana equivalent.