In this article Charles Altendorf outlines a method he developed to convert GIS data located in a geodatabase created in ArcGIS into open source data that can be used in QGIS.
geodatabase
GIS Data
GIS File Formats
Listed here are some of the more common file formats along with associated file extension for GIS file types.
GIS Data
Types of GIS Data Explored: Vector and Raster
This article discussions different types of GIS data in both vector and raster formats.