Microsoft® Word is probably one of the most common file formats used by translators today. Some other tools make you translate Word files in a separate application, but there are two big problems with this: (1) You may be used to working in Word and don't relish learning a new word-processing program; and (2) There are often subtle formatting differences when the translated text is put back into Word.
Felix lets you work right within MS Word. A simple click of a button or keyboard shortcut allows you to look up sentences, and register your translations as you work. There is no disruption to your workflow: simply translate the text as you normally would, and use Felix as it suits you.