A brand is the company name found on a particular model of watch.
Watches are complex products made up of components of different materials, primarily including the case, movement, strap, dial, and so on. Historically, each of these were produced by different companies or workshops before being brought together. The final assembler or retailer of the complete watch would put their own name on the dial, thus creating a brand name. Different watches from the same brand typically have model names, and may have other identifiers as well.
Over time, watch buyers began to prefer companies that produced most or all of the watch, particularly the movement ("manufacture"). This brought acclaim to brands like Rolex, Jaeger-LeCoultre, Longines, Omega, and A. Lange & Söhne. But many successful and admired brands still do not produce most of the components used, and even those "manufactures" often outsource some components, particularly the strap or bracelet.
The advent of watch trusts has had some impact on brands as well. Now that some, including the massive Swatch Group, own a dozen or more brands, there is some confusion about the true producer of a watch. Is Omega a "manufacture" even though ETA makes movement components for them? This is particularly confusing for companies that offer similar models in two different brands, as is the case for Rolex and Tudor.
Many brands have their own retail operations or boutiques. Conversely, some retailers have their own brands as well. Wempe models are built in Glashütte by Nomos, while Bucherer has the captive Carl F. Bucherer brand. In the USA, Tourneau has their own brand of luxury watches as well.
Watch brands are often bought and sold and can be re-started after long lapses. Many companies claim historical legitimacy for their brands with little regard for continuity of ownership or production. A. Lange & Söhne, for example, was re-started after the fall of the Berlin Wall in the same city and by a descendant of the original family owners, but the modern Nomos has no relation to the historic brand apart from their location.
So-called mushroom brands appear and disappear suddenly, selling watches built of other companies' components and providing little input into the final product. This was common in previous decades as well, with retailers and distributors inventing brands and applying them to completed watches for import or stock liquidation purposes.