Sometimes people will use what is called an 'impression mat' to add texture. I've also known people to use paper towels that have a design and rub them on the cake to add texture.
I'm not sure exactly which textures you are describing, but I have seen some cakes that are made with crusting buttercream and smoothed with the "Viva" paper towel smoothing method... except instead of using Viva brand paper towels you can use paper towels that have a design imprinted in them. HTH!
Yoy can also use a tool called a "comb" that you drag across the surface of buttercream or whipped icing to give it ridges or waves. On whipped icing, I've also heard on this site about using a large pastry brush to give it texture.
I am not sure what you mean by "texture" either. If it is a fondant cake I would say it is an impression mat or textured rolling pin. If it is buttercream it may be something like this cake? That is done with a long bladed bread knife with a serrated edge. You ice the cake smoothly, hold the knife in both hands at an angle towards your self, start at the back edge of the cake and with the teeth of the knife barely touching the surface of the icing move your hands in wide side to side waving motion till you get to the front of the cake. Use an icing spatula smooth any excess icing down onto the sides of the cake. Then hold the knife upside down with the tip of the blade of the knife touching the cake board just next to the cake, angle the blade in so the teeth are just touching the icing and move your hands in a quick up and done zig zag design, closer together than the waves on top of the cake. Check as you go around that the knife is straight so the zig zag lines don't begin to lean to the side.
http://www.cakecentral.com/modules.php?name=coppermine&file=displayimage&meta=allby&uname=ShirleyW&cat=0&pos=102
I am sorry I didn't describe it very well... but it is on this cake - I can't get it to attach as a pic
http://www.cakecentral.com/cakephoto_222013.html
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