Every language borrows words from foreign ones and adapts the pronunciation to match the sounds available in their language. Japanese is no exception to this, and if you speak English, you already know a lot of these loanwords, and will learn more as your studies continue. Katakana is the script system that is used in Japanese to write these loanwords. Katakana is also used in order to create emphasis the way that languages written in the Roman alphabet use italics to indicate emphasis on words. There are as many Katakana as there are Hiragana.
* Katakana "wo" is rarely used.