The short answer is that they are the same. 이따 is just a shortened version of 이따가. Both mean "after a while" or simply "later". For more details, 이따가 comes from 있다가, made up of 있다(to stay) and -다가(verb ending for "while doing one thing"). 있다가 was once used for both "after staying at one place for a while" and the simpler "after a while", but the latter purely temporal meaning branched out to 이따가. So 있다가 and 이따가 are now two distinct phrases, even though they have exact same pronunciation (some native speakers mix them up too). E.g. (the two words can't be intermixed below) - 여기서 두 시간 있다(가) 가자 = Let's hang around here for a couple of hours (before moving on). - 지금 바쁘니까 이따(가) 내가 다시 전화할게 = I'm tied up at the moment. I'll call you back later. Here's other -다(가) examples. - 이 일은 어려우니까 하다(가) 힘들면 쉬어 = This is a tough job, so take a break when it feels like too much. - 벌써 두 시간이나 걸었다. 여기서 좀 앉았다(가) 가자 = We've been walking for two hours already. Let's sit here for a bit.