Their documentations are simple and professional. But they don't mention too much about the relationship between these open source projects. When should I use which one? And which one is suitable for what scenario?
If you are a GIS developer who is familiar with these projects, can you explain?
In basic terms, GDAL is used for reading, writing and transforming raster data, while OGR can do the same with vector data. I am not as familiar with FDO, but it appears to be an API used to access (from database sources), manipulate and analyze all kinds of geospatial data, and relies on GDAL and OGR for those purposes.