Childless is the condition of ‘not having children.’ People may be childless for various reasons: infertility, medical complications, circumstances involving the partner being the most common ones.
The word childfree is a more recent term, referring to people who choose not to have children. They may decide to commit themselves to their career or other activities that may be difficult to pursue if a child has to be raised as well. Or they may simply not have the mentality required for parenting. There may even be financial, social or political reasons behind a person’s decision to remain childfree.
Knowing the difference is essential, because a person who is childfree may not prefer being referred to as childless, and vice versa, because ‘choice’ is an important factor at play here. Childless people may want children but may have have been left with no choice or possibility of having one. They may not like it if you imply that not having children is their deliberate decision. Likewise, a person who can have a child but has simply decided not to, may be offended if you imply that they are unable to have one.