The Developer Tab is an additional, hidden tab that can be useful when you write VBA code, especially if you prefer to work with the mouse rather than the keyboard.
The Developer tab is hidden by default. To show it in the ribbon, go to FILE >> Options >> Customize Ribbon and click the checkbox.
Now you can find the Developer tab as the last tab in the ribbon.
The tab is divided into several groups, such as Code, Add-Ins, Controls, XML, Modify.
Each feature in the code group can be found in the ribbon, with the exception of the Macro Security button.
The add-In is a program written in one of many programming languages. It adds new functionality to Excel. You can find two types of add-ins: Automation add-ins and COM add-ins.
In Excel 2013 you have two types of controls: Form Controls which are simpler and ActiveX Controls which are more flexible and allow you to control them from the VBA editor.
Since Office 2007 and the introduction of Open XML format, XML started to play a bigger role than before. That why Microsoft gives you additional tools to handle XML.
You can view or change document properties using this button. To access them, go to FILE >> Info >> Properties >> Show Document Panel.