Discussion forum is an emblematic computer-mediated communication tool. It is one of the oldest CMC tool and it is characterized by several paradoxical features, which can be observed in internet-mediated communication in general. This paper aims to define precisely this CMC tool and to show its interest for CMC Studies. Then it will be shown that communicative exchanges in forums can be analyzed through three paradoxes: they belong to interpersonal and mass media communication, they can be at the same time intimate and impersonal, they correspond at the same time to digital writing and standard written French. These analyses will be illustrated by examples from three prototypical French-speaking discussion forums: Doctissimo, Ados.fr and Fr.rec.boissons.vins.