Shapefile output from the ArcMap GeoProcessing wizard may have truncated field names.
For example, you have a layer in your map that has a field named WorldPopulation2001. If you use the GeoProcessing Wizard to clip this layer, the output layer retains all fields and values from the input layer, including WorldPopulation2001. However, the long field name is truncated to WorldPopul.
Shapefile attribute tables are stored in dBase format (.dbf); thus, dBase field names are limited to ten characters.
One exception is output from a Dissolve operation. In this case, ArcMap creates aliases in the output layer automatically.