In Salesforce, skinny tables are used to access frequently used fields and to avoid joins. This largely improves performance. Skinny tables are highly effective, so much so that even when the source tables are modified, skinny tables will be in sync with source tables.
Considerations for skinny tables:
Skinny tables can contain a maximum of 100 columns.
Skinny tables cannot contain fields from other objects.
For full sandboxes: Skinny tables are copied to your Full sandbox organizations, as of the Summer ’15 release.