Like Hiragana, Katakana is a syllabary used in Japan. While Hiragana is used for “Japanese-y”/non-Chinese type things — Japanese words, declensions, inflections, and pronunciation, Katakana is used mostly for transcribing foreign words and/or words with a foreign origin. (This is similar to the use of italics in English for bons mots from other languages.) It is also used for “sound effects”, technical terms, and occasionally for emphasis.
Like Hiragana, Katakana glyphs are sub-elements of Manyogana glyphs, but Katakana appears to have come from a more angular variant while Hiragana comes from a more cursive variant. There is a one-to-one correspondence between Hiragana and Katakana characters.
Vowel duration is semantically meaningful in Japanese, unlike English. (“Boooooooooriiiing!” might have a different emotional connotation than “Boring!”, but not a semantic one in English.) While in Hiragana, vowels with a longer duration are marked with another vowel (so “taa” would be “ta”+”a”), Katakana uses a symbol (bouten) which looks like a hyphen (so “taa” would be “ta” + “-“).