Learning is a social process. That is why it is extremely important to understand how students interact socially in online courses and how it affects the learning process. However, social aspects, understood as those expressions or comments that go beyond strictly academic interaction, i.e. the need to carry out group work, are not clearly defined. Researchers have proposed different models of categories to observe or measure social aspects. This paper contributes to this field through addressing the categorization of social expression in online groups through a qualitative research procedure. Specifically, 19 indicators have been identified and organized into 4 categories: formal, attitudinal, emotional and informal. The findings suggest that those indicators related to formal and attitudinal aspects appear more often than emotional and informal ones. Different profiles (Psychology or Computer Engineering) as well as different levels of experience in online learning (beginners or experts) have also been analyzed, concluding that Psychology students turn to social expressions more often than Computer Engineering ones. As students progress in their undergraduate studies, social expressions are perceived and used to the extent that they ease the learning process. (Contains 1 figure, 1 picture, 7 graphs, and 5 tables.)