Wetbrushing are a techinque of getting surfaces to look more 3d and for weathering of surfaces like stone.

If you have a plain painted surface (textured is best or formed like stone or brick).

Wet brushing - if you use a dark coloured paint like dark brown, green or black depending on desired finish type. mix paint with about 90-95% thinners so when you paint over a newspaper it just darkens in (still read print) paint over the surface in a flat position and allow to dry.  The darker tinge will sit in low areas and drain off upper surface a little. Darken up if desired then:-

Drybrushing using a much lighter colour like brick cream or beige use a ting bit of paint and rub into bristles  (on a cloth or similar) until when you gently paint over the surface it highlights the upper surface detail. Build up layers until desired finish is reached Using different shades can help if you don't want it uniform over an area.  Repeat wetbrushing and drybrushing until you get things how you want.

Varients on these techniques can give weathering on buildings, rust streaks from Iron or steel, verdigree on copper roofs and even pluracocci from guttering downpipes (water leaks from joints) this all adds the the life of the model.

Brickwork is similar except the mortar is beige/cream so wetbrush with light colour then drybrush with terracotta and weather with the black wetbrushing.

Its good fun playing with these techniques as it not only gives realism to the model but can be a much faster way of painting large areas like brickwork or stone.