The Union tool takes the input of multiple polygon layers and combines all the information into a single feature class that contains all the data from the input layers (typically two or more). It is important to remember that this tool only works with polygons. It can't be used with points or lines. If you need to perform this type of analysis on points or lines, you will need to use the Identify tool.
You might want to use the Union tool if you wish to determine how much of each parcel was in a floodplain area and how much of each parcel was not in a floodplain area, as shown in the following screenshot:
As you can see, the result is a new layer or feature class that contains the attributes from each part of the floodplain that overlaps part of each parcel and the parts of those two layers that do not overlap. Again, your original inputs are still intact.