The designation ER simply means Extended Range. It's a Code used by Boeing and other manufacturers, it is not issued by the FAA. ETOPS, however, is a standard set by the FAA.
There are not many differences between a Etops and a non-Etops twin. However, there are minor equ. requirements such as more life jackets, etc.
Etops is actually more of an Airline rating, not so much the plane's. Even the Airbus 32X family are certified for 120min ETOPS. Etops requires the airline to have stricter(sp?) rules on the aircraft's maintance, one good example is that a mech. will only replace an oil filter on one engine, while another mech. will replace the other's. This elimates the chance the the mech can do the wrong thing twice, therefore leaving a twin engine-less.
Boeing 777's are 180min out-of-the-box, and there are rumors that the recently launched LRs will be 207min -out-of-the-box, which currently, is only granted on a case-by-case basis, and only with the right airline/plane/engine combo.
Hope I got it right,