I know that for creating texture on their paintings many digital artists use texture overlays or fancy custom brushes. But how to achieve similar effects using only simple brushes? Are there some techniques? How can this be achieved using traditional media, like acrylic or oil paints?
You can get paint gels that are made to be mixed in to various types of paint to thicken it, add texture, make it more suitable for 3D structuring and / or add matt, gloss etc.
Here's a few from http://www.eckersleys.com.au/ (no affiliation or recommendation, just the first search result)
Or, the DIY / low budget approach is to just either: