The development files are the include files needed to compile C programs, the PKG-CONFIG .pc files, the static libraries and a demo program (like gtk-demo)
.
|-- bin
| `-- gdaui-demo-5.0
|-- include
| `-- libgda-5.0
| `-- libgdaui
| [...]
`-- lib
| |-- libgdaui-5.0.a
| |-- libgdaui-5.0.la
| `-- pkgconfig
| `-- libgdaui-5.0.pc
`-- share
`-- libgda-5.0
`-- demo
[...]