Originally these verbs were different. "Класть" meant "to put inside" (cf. Клад), while "Ложить" meant "to lay on the surface" (cf. Ложе).

But nowadays this difference is completely lost, and we have what we have now: класть is imperfective, and prefixed form положить is perfective. You shouldn't say покласть or ложить anymore. That's it.

Originally these verbs were different (not sure, whether it was in Old Russian, or in proto-Slavic). "Класть" meant "to put inside" (cf. Клад), while "Ложить" meant "to lay on the surface" (cf. Ложе).

But nowadays this difference is completely lost, and we have класть as imperfective, and prefixed forms, such as положить as perfective. You shouldn't say покласть or ложить anymore. That's it.