If you're self_employed, your after_tax income equals your gross income less your business expenses, such as the cost of your laptop or airfare to conferences, as well as the amount you set aside for taxes. You're responsible for remitting your own quarterly estimated tax payments to the government because you don't have an employer to take care of it for you.
You might choose to break down your expenses even further, between things you need to have and things you want to have. For instance, if you drive to work every day, gasoline probably counts as a need. A monthly music subscription, however, may count as a want. This difference becomes important when it’s time to make adjustments.