A Personal Geodatabase is an ESRI-specified GIS database, in Microsoft Access (.mdb) format.
A personal geodatabase is a Microsoft Access database that can store, query, and manage both spatial and nonspatial data. Because they are stored in Access databases, personal geodatabases have a maximum size of 2 GB. Additionally, only one person at a time can edit data in a personal geodatabase. Personal geodatabases are made up of nine system tables plus user data.
Allowable Dataset types:
• Feature class
• Feature dataset
• Mosaic dataset
• Raster catalog
• Raster dataset
• Schematic dataset
• Table (nonspatial)
• Toolboxes
Allowable Feature Dataset types:
• Parcel fabrics
• Feature-linked annotation
• Geometric networks
• Network datasets
• Relationship classes
• Terrains
• Topologies
Personal geodatabases can also contain domains, use subtypes, and participate in checkout/check-in replication and one-way replicas.
The usage of personal geodatabases is now deprecated in lieu of the more efficient file geodatabase. File geodatabases have a 1 TB storage limit (vs 2 GB for personal geodatabases) and also support multi-user editing of feature classes.
For more information: http://help.arcgis.com/en/arcgisdesktop/10.0/help../#//003n0000007n000000