As St. Thomas puts it, "those acts are properly called human which are voluntary, because the will is the rational appetite, which is proper to man." In ST I-II, 6, he discusses what makes an act voluntary, and what might make what would otherwise be a voluntary act involuntary.
In Article 1, he distinguishes voluntary acts and natural acts (his example of a natural act is a stone falling). Both are acting upon a principle that is intrinsic or internal to the actor, but in a voluntary act there is knowledge of the end or purpose of the act.
In Article 2, he argues that irrational animals can act voluntarily, with "imperfect knowledge" exercised "through their senses and their natural estimative power."
In Article 3, he identifies two ways in which you can speak of something as "voluntary" even when there is no corresponding act: you can perform the internal act of willing not to act (e.g., "I think I won't help him up"); or you can fail to perform even the internal act of willing in a situation when you can and ought to act. Thus, though St. Thomas doesn't mention it here, we can speak of "sins of omission," even understanding that each sin is a chosen act.