Fantastic Recent Progress in Low-Mass Objects
Our understanding of the stellar and sub-stellar objects that lie between the traditional bottom of the stellar Main Sequence, M-dwarfs, and the gas-giant planets found in solar and extra solar systems is progressing at a fantastic rate. This progress has been fueled by follow-up spectroscopy of candidates identified by state-of-the-art NIR surveys such as 2MASS, and by sophisticated theoretical models linking the bottom of the main sequence to planets.
Two new classifications have been added to the spectral classification sequence.  Later than M-dwarfs, the newly added 'L' and 'T' dwarfs represent objects that straddle the stellar/sub-stellar boundary and below. The 'L' class spans Teff~2000-1400K. The stellar/sub-stellar boundary is expected to be ~ L 4. Below this point sustained fusion of hydrogen is impossible because of insufficent mass.  (Kirkpatrick et al.)
'L' dwarf spectra are markedly different from the traditional metallis-oxide (TiO and VO) dominated spectra of 'M' dwarfs. With cooler atmospheres, the TiO condenses into grains and VO solidifies. The dominant spectral features become metallic-hydrides (FeH and CrH) and alkali atomic absorption lines.
'T' dwarfs are cooler yet and are defined by their telltale methane absorption features.  These objects are cool enough that methane can form in their atmospheres.  Jupiter, the archetypical gas-giant, also has methane. The latest T-dwarf discovery has Teff ~750 K (Burgasser et al.)
Apart from the fact that 'planets' form from circumstellar material and that field 'stellar and sub-stellar objects' form by gravitational collapse of material, which give them important kinematic and enviromental differences, fundamentally these objects lie on the same continuum of mass and temperature. The burgeoning field of low-mass objects is rapidly filling in our understanding of this continuum.