Katakana is one of the three character systems(the others are hiragana and kanji) in Japanese. There are 46 characters in katakana, but there are also symbols that can be applied to them that change their sounds. These symbols are dakuten and handakuten. Dakuten, like in "が", are the two lines slightly above the character, and handakuten, like in "ぱ", is the circle. The characters, "ka, sa, ta, and ha" only have dakuten and for handakuten, it is only "ha" that may have it.
Katakana is often used to represent foreign loan words. What does this mean? Well, an example would be "ビデオ", or in romaji, "bideo", which means video in English. It sounds similar to video, but that is only the case for some foreign loan words. But, you might say that there is already "動画", which is "どうが" or "douga" in romaji, which also represents "video" in English. The english loan word is sometimes used more often, rather than the kanji-based word.
Another use for katakana is for some emphasis or a more unique way of writing out words. It is less used, so when encountered on a word usually written in hiragana, it can provide emphasis on that word. An example would be "メガネ" or "megane", in romaji, meaning "glasses" in English. Emphasis is placed on this word. It could also be "眼鏡" in kanji form.
Test your katakana ability with this hiragana and katakana test!