The Top Clients report will allow you to view which hosts on your network have been the most active accessing servers on your internal an on external networks. The various report filtering options can be used to focus the reports on the downloading activity that is of interest to you.
For example to determine the top clients on your network that are accessing external websites simply access the Top Clients report at
Bandwidth -> IP -> Top Clients
Specify port 80 in the Server Port field.
In the Server field you need a filter to record accesses to only Server IP addresses which are not on your internal network. If your internal network has a subnet address of 10.40.0.0/16 then enter !10.40.0.0/16 in the Server field.
This report can be used to view the Top Downloaders for particular network services.
You are correct in saying that the term "client" does refer to a host receiving traffic from a "server