Its a hammer with the cocking spur cut off. This makes the gun more snag proof. Most any 'smith can cut the spur off for you.
The only lookout is to make sure the hammer isn't lightened too much, causing misfires.
Most factory bobbed hammers are also altered to double action only (DAO). They can't be cocked. Gunparts sells factory bobbed DAO hammers for some S&W models.
Years ago, it was very popular to bob the hammer on the Colt Detective Special. If the top of the hammer was grooved, the gun could still be cocked, if you were VERY careful.
Most revolver manufacturer's offer bobbed hammer guns. I know Ruger, S&W, Taurus, and others do. As I said, the big advantage is that the gun is less likely to get snagged in your clothing.