Difference Between The Transisor Denotations V(th) and V(tn)
V(th) is general whereas V(tn) is a little more specific.

V(th) is just the threshold voltage.
V(tn) is threshold voltage of an nmos.
V(tp) would be the threshold of a pmos.

Note that BJT's are usually spoken of as current devices so they aren't usually talked about as having a threshold voltage. Incidently, they often have a Vbe (voltage base-emitter) of about 0.7V. BJT's have a \$V_T\$ which is the thermal voltage which is about 25mV: http://en.wikipedia.org/wiki/Boltzmann_constant#Role_in_semiconductor_physics:_the_thermal_voltage  

Lastly, don't be surprised with typos in textbooks on Vth, Vtn, and Vtp. Those always make it even more confusing...