Under the “Table of Contents” the various layers of the map are shown. Right click on the desired layer, and then go to “Open Attribute Table”.
The Attribute Table opens. It shows what kind of data are stored in the data file. You may also process this data and add data (see below).
You can also see what type of data is stored: Go to “Fields” via options and under “Choose which fields will be visible” you choose and click the information you wish to have a look at. Here you can see which “Data Type” this field has:string, text, integer, double.