Along with my travel paint set, I am frequently asked about these brushes. These are water brushes, or aqua brushes as they are sometimes known. The brush tip unscrews from the barrel and you can fill them with water. Squeeze them to draw water to the tip, which you can then use to paint. To clean, just squeeze again to flush the tip on some paper towel. It's a great way to paint on the go without any setup.
But you don't have to fill these with just water. Paint or diluted ink works well too. I used diluted sepia ink when I sketched at the museum for this post. Diluting the ink with water meant I could build up the colour for different shading values.
The ones above happen to be Sakura. But there any many brands (Pentel, Niji, Koi, Derwent to name a few) and are all quite good.