Katakana is usually used in the following ways:
* to transliterate foreign loan words from English, Chinese and other languages such as television (テレビ - terebi), radio (ラジオ - rajio), fried rice (チャーハン - chaahan), Chinese noodles (ラ-メン - raamen), part time work (アルバイト - arubaito) etc.
* foreigners', except Chinese, names are written in katakana such as Smith (スミス - sumisu), Brown (ブラウン - buraun) etc
* for the names of foreign countries, regions and cities such as "America" (アメリカ - amerika), England (イギリス - igirisu), Vietnam (ベトナム -betonamu), London (ロンドン - rondon), Asia (アジア - ajia) etc.
* in advertising and on signs for emphasis, here katakana acts much like italics, such as garbage (ゴミ - gomi), spectacles (メガネ - megane), pervert (チカン - chikan), coffee (コーヒ - koohii) etc
* animal, fruit and plant names are often written in katakana such as monkey (サル - saru), dog (イヌ - inu), apple (リンゴ - ringo), persimmon (カキ - kaki) etc
* onomatopoeia, words representing sounds or movements, for example, bow-wow (ワンワン - wanwan), lick (ペロペロ - peropero), scratch (ポリポリ - poripori), squeak (キシキシ - kishikishi)
* some Japanese company names are written in katakana, such as Toyota (トヨタ), Sony (ソニ), Honda (ホンダ) etc
Katakana was developed in the Heian Period (794-1192) from parts of Chinese characters as a kind of shorthand, supposedly by students studying Buddhism.
Katakana, hiragana and kanji (Chinese characters) all in the same sentence!
Loan words such as "Museum" are written in katakana as in this sign for the Ryukoku Museum in Kyoto