A Spatial join is a GIS operation that affixes data from one feature layer’s attribute table to another from a spatial perspective. Spatial joins begin by selecting a target feature and comparing it spatially to other feature layers. The target features will inherit the attributes from the other features if and only if the two features share the same spatial reference. Spatial join is a way that data from one feature class can be easily added to another feature class. Most of the time a new feature is created with combined data from one of the original feature layers. This occurs, for example, when using ArcMap's Spatial Join tool. It will not update the target area but will create a new feature class.
Spatial Join Types
A spatial join uses one of two types of relationships: a one-to-one relationship or a one-to-many relationship. A many-to many-relationship is not possible in a spatial join. The relationship of the spatial join is between the Target Features, the table that you are joining to, and the Join Features, the table you are joining from. 
One to One Relationship
In a one to one relationship, the target features table will have the same number of rows as the final table after the spatial join is executed. If the join features table had more rows than the target features table, the data from the join features table will be clustered to fit into the corresponding row values of the target features table.
One to Many Relationship
A one to many relationship occurs when there is no desire for possible clustering of the data from the join feature table. Instead, the final table will accommodate the possibility of having multiple entries from the join features table that may correspond to a single row from the selected field in the target features table. This is achieved by having a final spatially joined table with the possibility of repeating the same values of the target's feature table within the selected field that was used to join the join features table to the target's feature table.
GIS&T Body of Knowledge
This topic is covered by section DN2-4 of the GIS&T Body of Knowledge.
- ArcGIS Help 10.1, Spatial Join(Analysis), http://resources.arcgis.com/en/help/main/10.1/index.html#//00080000000q000000
- Spatial Join (Analysis), ArcGIS Desktop Help 9.2, Accessed 9 October 2012.
- Spatial Join (Analysis) ArcGIS Desktop Help 9.2 Accessed 9 October 2012
- Joining the Attributes of Features by Location ArcGIS Desktop Help 9.2 Accessed 9 October 2012
- About Joining and Relating Tables ArcGIS Desktop Help 9.2 Accessed 9 October 2012
- Adding Attribute Data (join; spatial join) Duke University Libraries Accessed 9 October 2012
- Spatial Join is included in the Wiki.GIS.com Glossary GIS Glossary/S