PTO is the general acronym for the many groups (PTOs, PCCs, HSAs, etc.) that choose to remain independent. PTOs are free to write their own bylaws and they can either: a) not charge dues at all (a nice touch to encourage involvement); or b) keep whatever dues they do charge at their school.
While PTA is the most well-recognized acronym to the general public, these days more than 75% of K-8 parent-teacher groups are actually independent PTOs. Of the more than 112,000 K-12 schools in the United States, fewer than 22,000 of those still have formally-affiliated PTA units.