We present in this article the concept of digital coach. More than a traditional assistance, the digital coach allows the user to improve his/her capabilities, by guiding him/her, step by step, particularly in a vocal way, and by stimulating him/her thanks to personalized information. Those are prepared specifically according to each user (texts and synthesis voices in his/her language) and are exploited by an application running on a mobile device. That makes it possible "to be coached", digitally, and individually, during the activity, and not preliminarily and in a generic way. The application fields are diverse (sport, cooking, doityourself, culture, tourism...). A definition and a first taxonomy of digital coaches are proposed in this article. The concept is then illustrated thanks to a prototype of a coach embedded in a mobile device.