an OMS is just an order management system, traditional. the EMS, or execution management system is being "installed" to keep up with increases in quote and trade traffic. They are for algos, yes, but also to cancel and replace orders in the new "fast" markets.
Basically, OMS is a front end, EMS is more piping or behind the scenes. Some will say what appears to just be an OMS is so much more.....A lot of cloudiness exists as to the usefulness and/or where pre trade and post trade applications fit in all this. That's all just pretty much a land grab for more bluishness.
I'm sure someone else will have a different definition, but that's my take.
EMS conceptually are built for trading more than just order management....