I've generally seen brown dwarfs depicted as more massive and slightly wider Jupiters in varying colors with banded cloud structures, sometimes hot enough to be visible glowing. I've also seen red dwarfs generally depicted as smaller and redder suns with sunspots and granules, and Wikipedia mentions that red dwarfs are prone to large starspots and flares. My question is where is the transition region between them and what happens there? Can there be brown dwarfs with starspots? Red dwarfs with bands?
A "star" has significant hydrogen fusion occuring in its core. This energy prevents collapse and allows the star to reach a stable position state.
A Brown dwarf does not have significant hydrogen fusion. Its core is heated by the release of gravitational energy (the Kelvin–Helmholtz mechanism).
The transition between brown and red dwarfs is at around 80 Jupiter masses. At the transition it can be hard to distinguish brown and red dwarfs, especially when they are young. It may not be possible to tell the difference between a young star at 85 Jupiter masses, and a young brown dwarf at 75.