Labeling is a way to add descriptive text to features on a map. Labels are dynamically placed, and label text strings are based on feature attributes.
You can use dynamic labeling for all features in a layer, or you can use label classes to specify different labeling properties for features in the same layer. For example, in a layer of cities, you can label those that have a population greater than 100,000 with a larger font size and those that have a population less than 100,000 with a smaller font size.
Label symbols
You control the appearance of dynamic labels by changing the label symbol. The label symbol is a text symbol that is used for all the labels in a layer or label class. Text symbols have basic properties, such as font, size, and color. You can also work with more advanced text symbol properties to add callouts, leader lines, shadows, halos, and other effects to your labels.
Note:
When labeling, text size can be overridden by scale-based label sizing.
To change the appearance of labels, choose a text symbol from the Text Symbol gallery on the Labeling tab, change the properties of the existing label symbol through the Text Symbol group on the Labeling tab, or open the Label Class pane and click the Text Symbol tab.
For instance, on a map of European capital cities, if you want to use a bold font, you can change the properties of the existing label symbol to make it bold. Or, since the ArcGIS 2D system style contains a number of text symbols that are useful for labeling, you can choose the Populated Place (Capital) symbol from this style and use it to label the European capitals.
To reuse any text symbol for labeling, click the menu button 
on the Text Symbol tab of the Label Class pane, and choose Save symbol to a style. You can then use the saved text symbol for labeling or any other purpose.
Learn more about saving symbols to a style
Caution:
The following text symbol properties are not used when labeling:
- Angle—You can specify the orientation of labels through the label placement properties.
- Vertical Alignment—The vertical alignment of labels is always bottom.
- Horizontal Alignment (Maplex Label Engine only)—You can specify the horizontal alignment of labels through the placement properties.
Caution:
Changing X Offset and Y Offset for a text symbol is not recommended. The label engine first places the labels using the labeling parameters and symbol settings and then applies the offset values, potentially resulting in overlapping labels.
Placement styles
Another option for changing the appearance of labels is to choose a label placement style. A label placement style consists of label placement options. You can change the positioning of labels by choosing a label placement style from the Label Placement Style gallery on the Labeling tab. The ArcGIS 2D style contains a number of useful label styles for labeling points, lines, and polygons.
From the Label Class pane, you can set the individual labeling placement parameters for label classes. To open the Label Class pane, click the Label Placement Properties launcher 
in the Label Placement group on the Labeling tab.
See Standard Label Engine and Maplex Label Engine for more information about label placement.
To reuse any set of placement parameters for labeling, click the menu button on the Position tab of the Label Class pane, and click Save Position to Style. You can then use the saved placement style for labeling. It is accessible from the Label Placement gallery on the Labeling tab.