The PKK are a Kurdish political party in Turkey. They're advocates for an independent Kurdish state separate from Turkey. Turkey is a NATO ally of the USA and they consider the PKK to be a terrorist organization because, well, they have been many times throughout modern history. They let off bombs that target civilians, fire weapons on police and act just as you'd expect militant separatists to act. That means the USA considers the PKK to be a terrorist organization, too, despite the fact that they have acted as an ally of ours against ISIS and, yes, they were also instrumental in the rescue of the Yezidis, who are a Christian Kurdish sect, when they were trapped on a mountain top by ISIS forces. I would argue that the KRG, the Iraqi Kurdish party led by Barzani, was more instrumental to their rescue, but OP's general point is fairly correct, though it ignores the car bomb that went off in Turkey this weekend and the end of the ceasefire the PKK had with Turkey over the last couple years.

The situation is getting very sticky. As things stand right now, the USA doesn't communicate directly with the PKK - only the YRG and the KRG - but the PKK does much/most of the legwork in Syria. That means they're getting most of the US weaponry that's headed that way. Turkey is afraid the PKK is going to push ISIS south, which they have, and then the PKK and the YRG (Syrian Kurds) will start pushing north through the Kurdish areas of Turkey and move for their own state.

Anyway, it's a very convoluted situation. The PKK have acted like terrorists. BUT they're also helping a lot of people right now who were completely helpless not six months ago and their assistance is lining up well with Western interests. They're certainly better than Assad, so they've got that going for them right now, but they won't have that to their advantage forever and they're rightfully capitalizing on the moment.