Information and communications technologies offer unprecedented opportunities for innovation in educational practice. Among all the educational technological tools, online discussion forums represent one of the most extensively adopted educational media in higher education. Yet a review of the literature indicates a lack of empirical studies investigating how learners utilize this tool to afford their language learning. This study aims to address this gap in the literature. Using thematic analysis, postings of 20 second language (L2) learners in a peer-moderated online discussion forum were analysed. Qualitative thematic analysis of the data revealed that the online discussion forum facilitated the co-construction of subject matter knowledge and enhanced learners' critical understanding. The postings also showed that during the interactive, collaborative inquiry, students encouraged and helped each other emotionally and academically which helped foster group affiliation and learner autonomy. These findings suggest that the online discussion forum has many affordances which are conducive to effective learning. The paper concludes with pedagogical implications and suggestions for future research.