Similar to Layer Styles, Text Styles allows you to denote a specific combination of typography into a shared group. This is important when designing applications and websites as you are wanting your design to share elements to ensure that everything is consistent. You don't want a developer to have to make 50 instances of a typeface, but instead have defined Text Styles that are used strategically throughout the design. You are able to do this by clicking "Layer > Create Shared Style". If you need to update your style at any point, click the Update button. If you need to "undo" an update push for your text style, hold down option and a rever button will appear in place of the update. Sketch makes it easy to attribute a new line of text to a Text Style, and allows you to very rapid insert a new layer using the Text Styles that have been defined.