The train types could be confusing but not too difficult to understand.
You will mainly find EC (Eurocity), IC (InterCity, also ICE for InterCityExpress, mainly for Germany), IR(InterRegio), RE(RegioExpress), R(Regio) and s trains. In short, EC is like IC but with intertional stops, they stops only at big stations. IR RE stops at also smaller stations, R and S will also stops at commuter stations. Special category is the PE, it means Panorama trains, usually you need also book seats for those, famous example would be Glacier / Bernina Express.
From Interlaken most likely you will go to Italy via Brig, you will make your change at Spiez for the EC train to milano centrale. Though it will be a pity if you really want the best view on this route, as the fast trains like EC IC will use the lötschberg base tunnel, it cuts lots of travelling time but also you will miss all the good view Bernese Oberland can offer you with. If fast travel to Milan isn’t the top one priority I suggest you to use the Regio service by BLS from Speiz to Brig (on SBB it is marked as RE), you will travel along the original mountain route of Bern Lötschberg Simplon, it is truly beautiful and underrated as most tourists don’t know about it. From Brig you can change to the EC for Milano.