Column Totals are a great way to summarize data within 1 field of a record, in grid view. Depending on the type of field, you will have different options of how you want to total the column:
Qualitative Fields such as: Text fields, Select fields, and Title fields will allow you to summarize: Empty (Quantity), Filled (Quantity), % Filled, % Empty.
Numerical Value Fields will allow more summing capacities including: Empty (Quantity), Filled (Quantity), % Filled, % Empty, Sum, Lowest, Highest, Average, Median, Standard Deviation, Range
Date fields will allow you to summarize based on: Empty (Quantity), Filled (Quantity), % Filled, % Empty, Earliest Date, Latest Date, Date Range.
Status fields have their own special Column Total that gives you a real time visual makeup of your status fields.