Difference between thermal reactions and photochemical reactions In a photochemical reaction, everything starts with absorption of a photon. I.e. a ground state reactant is excited to the first excited state. Sometimes the reaction barrier on the ground state surface is too high; perhaps lower barriers exist in other dimensions that lead to undesirable products or dissociation. Perhaps the relevant barrier is much lower on the excited state surface, which makes the reaction possible in the excited state.

Is the difference, then, that thermal reactions take place exclusively on the ground state surface, while a photochemical reaction takes place, at least in part, on the excited state surface?
Yes, you are right thermal reactions take place exclusively on the ground state surface, while a photochemical reaction takes place, at least in part  on the excited state surface. 

But there is another difference between them as
photochemical processes offer an advantage over thermal methods by forming thermodynamically disfavored products. Therefore, they overcome large activation barriers in a short period of time, and allow reactions that were inaccessible by thermal processes.