Usually when the companies offer free online stock trading, you will get access to a free membership that makes revenue by selling ad space or by charging commissions on trades. This is either about making your stock trading full of pop-ups, flashy ads, and biased information; or this is about paying excessive fees every time you make a trade.

There is another group of "free online stock trading" companies that charge membership fees, but do not charge for the execution of trades. In cases when you are going to make a lot of small stock trades each month, then paying just monthly fee is a lot better than paying fees when per each trade. In the cases of few large trades per month the model of paying per trade is much better than monthly fees for the membership.

As you can see, even with these free models, it is possible to choose the one which is adequately ok in terms of payments for you.