One is a system, the other is an algorithm
Lead routing
Lead routing is the whole decision layer that answers a single question: who should own this lead? It can consider source, product interest, territory, language, deal value, existing account ownership, working hours, team capacity and anything else you can express as a condition.
Round robin
Round robin is one distribution method inside that layer. It hands leads to a pool of reps in turn so everyone receives a similar number. It answers fairness, not fit. Every round robin is lead routing; most lead routing is not round robin.
