There are 2 kinds of load balancing: transport-level and application-level load balancing.
The former load balancer deals with every tcp connection. The load balancing decision is made on the TCP level (Layer 4 of the OSI Reference Model). One of the routing mode Network Address Translation (NAT) routing.
The latter load balancer deals with every HTTP request. The load balancing decision is made on the content of the HTTP request.
Ref: Server load balancing architectures, Part 1: Transport-level load balancing Server load balancing architectures, Part 2: Application-level load balancing