A PTA is a parent teacher group that has members who pay dues to become affiliated with the National PTA. So if you are part of a PTA, you have dues that you pay to the National organization, a state organization, and sometimes a local organization--as a result, PTAs at individual schools charge dues to cover those (at a minimum). Those groups who are PTAs have standard sets of bylaws dictated by their state chapter. If you go to the National PTA website, you will see that they have a whole advocacy section where they take positions on education related issues on behalf of its members; those are in addition to positions taken by various state and local PTAs.

A PTO is a generic term for a parent teacher group that is not affiliated with anyone. Some PTOs charge dues, some don't. There is no one standard set of bylaws used by PTOs. Some are called PTOs, some are called HSAs, PTGs, and any one of a number of other acronyms. When it comes to the day to day tasks done within an individual school, you probably won't find a whole lot of difference among any of the groups, though.

What is a PTO exactly and how is it different than a PTA? Our school we have what we call a PTM (parent and teachers for Mckinley). They seem to all be the same thing, but some of the info I have read on this site suggest that at PTO or a PTA are different since the word DUES keep being brought up. We don't have dues or fees to be part of the PTM, we just have volunteers.
Thanks for any clarifications.