It's the distributed networking equivalent of programming with managed versus unmanaged memory.

With Akka you manage objects actively, much like memory in C, with Orleans your objects behave much like normal Dotnet/Java objects, they are just distributed and must use async methods. Orleans adds a layer of management, where it will tear down and re-instantiate objects as required, distributes them in the cluster, etc. As a programmer you don't have to worry about it, it's all managed by the runtime.