Beyond modules and depth, your company carries a set of feature switches: capabilities that sit inside the modules you already have. They are set by your company administrator on the company record (System Setup → Companies) and can be changed at any time — on its Features tab, which groups them exactly as the setup wizard's Features step does; the wizard is where they are first chosen, preselected from your profile's recommendations. This section explains each group by what it does — what appears, what changes, what it costs you in effort — so you can decide with your eyes open. The mechanics of editing the company record are covered in the Implementation Manual.
The standing advice of this manual: leave a switch off until you can name the problem it solves. Every switch below trades a little simplicity for a capability; §0.7 shows how companies grow into them.