http://www.huffingtonpost.com/2012/10/17/tab-for-a-cause_n_1971515.html
And here is an answer from the Tab for a cause team:
There are a couple banner advertisements in the corner each time you open a New Tab. We get paid for these advertisements, and then give all the profits to charities that partner with us. Tab for a Cause is free to use and always will be.
No matter what our costs are, we give at least 30% of our revenue to charity. Historically we’ve given upwards of 90%. We use the rest of the revenue for two purposes: (1) to cover costs, which include hosting/server costs and food for our team; and (2) to reinvest in getting more Tabbers so that we can give more to charity in the long term.
(Source)
On the actual page, you can choose which charity you can donate to. Here's a picture of what it looks like:
I hope this helped! Let me know if you've got any questions and I'll try to answer them the best I can.
u want commissions? u better pm me boi 
