Watercolor (watercolour) is a painting method in which the paints are made of pigments suspended in a water-based solution. Terms aquarelle or watercolor refer to both the medium and the resulting artwork.
Western aquarelle mediums are re-soluble after they dried. In childhood, I used a lot of opaque gouache, and disliked traditional watercolor. I have developed my own way to use transparent watercolor much later.
Traditional Eastern aquarelle paints tend to be non-resoluble after they are applied to paper - I used them only for a short period.
Professional watercolor paints I use are made with artist grade permanent pigments and gum arabic. I avoid additives like organic dispersants, fillers, or honey, and therefore I am limited only to a few expensive brands.
In the transparent watercolor technique pigments are applied in a manner allowing light to penetrate the layers of glazes and reflect back from the white surface below, and the texture of that rag or paper can be seen through the layers of paint.
Opaque watercolor, gouache, is a type of non-transparent water media, which I mix by just adding white pigments (titanium dioxide or zinc) to transparent watercolor paints, or just by applying them in opaque manner.
I do not use commercial gouache paints because the manufacturers use lots of additives, premix pigments, or do not provide the level of lightfastness required for fine art.
Gouache I use primarily in the top layers of watercolor paintings. In the past, I used it for studies for oils and as a stand-alone medium. I absolutely loved gouache as a child.