ArcMap Desktop Introduction: Combining Features and Layers

• Dissolve: A tool that combines multiple features within a layer into one based on a common attribute value. Online help.
• Merge features: Must be done in an editing session.  Can combine selected features within a layer. Online help.

You can combine layers (e.g., if the tracts for each county are separate layers and you need a single tract layer for an entier metro area) to get one layer.

• You can use the  Merge tool.  The result is a new layer without modifying your original layers. Online help.
• You can also use the Append tool to simply add the features in a second layer to your target layer. Online help.