The main difference between using Ribbon and a Load Balancer is the location of the load balancing logic.

Ribbon is a client-side load balancing library, which means that the load balancing logic is embedded in the client application, and it is responsible for selecting the appropriate service instance to handle each request. When a client application using Ribbon needs to make a request to a service, it contacts a service registry (such as Eureka) to obtain the list of available service instances, and then uses a load balancing algorithm (such as round-robin or weighted random selection) to choose which instance to send the request to.

On the other hand, a Load Balancer is a server-side component that sits between the client application and the service instances, and it is responsible for distributing incoming requests to the available service instances. When a client application makes a request to a Load Balancer, the Load Balancer decides which service instance to forward the request to based on the load balancing algorithm it is configured with. The client application is not involved in this decision-making process, and it simply sends the request to the Load Balancer.

So, the key difference is that with Ribbon, the load balancing logic is part of the client application, while with a Load Balancer, the load balancing logic is centralized in a separate server-side component. Ribbon provides more flexibility to the client application, as it can choose the load balancing algorithm and make dynamic load balancing decisions based on factors such as server health and response time. Load Balancers, on the other hand, are simpler to set up and maintain, and can be used to balance load across multiple client applications that are accessing the same set of service instances.

In summary, both Ribbon and Load Balancers are useful tools for balancing load across multiple instances of a service, but they differ in their approach and the location of the load balancing logic.