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I believe in Cisco speak, EtherSwitch service modules are modules that plug into the service module port on ISR routers. (For ISRs, I recall there are also small switching modules that connect into the ((E)H)WIC ports.) The module often provides its ports local L2 switching, and depending on the module, might also provide local L3 routing, otherwise the router provides L3 routing for the module's ports.
"Ordinary" Ethernet port modules, for the 4500 or 6500 series, are just considered line cards. However, the 6500 supports other services modules, such as firewalls, wireless controllers, packet analysis, etc.
