When to use geotextiles?

Geotextiles are used to separate layers of soil/gravel/sand etc. so that different layers won't mix and thereby lose their specific abilities (e.g drainage). It's typically used when constructing roads and infrastructure where primarily the drainage capacity of the soil is important, but also for buildings and other constructions.

In comparison, geogrids are typically used to fixate a layer in a soil construction, for instance gravel, so that it will stay in place and be able to withstand higher loads. It is useful to increase the capacity and prolong the service life of especially dirt roads.

For images and an overview of some common brands of geotextiles (and additional information if you know Swedish), see:

http://www.byggros.com/sv/produkter/vag-och-anlaggning/geotextilier