One of the ingredients of a recipe is Greek yogurt. May I replace it with plain yogurt? If I do it, what is the difference that I would immediately note?
Greek yogurt has more fat than a "normal" yogurt, about 10%.
Further an original Greek yogurt is made from sheep's milk since there aren't many cows in Greece. This might taste a bit odd for people used to cow's milk though...
When buying Greek yogurt made from cow's milk I recommend you look out for the native brand ΦΑΓΕ.
Greek yogurt is thicker. You can turn not-so-greek yogurt into it by letting it strain. Put cheesecloth into a colander, dump yogurt in, and allow to sit. Not too long, or you'll accidentally achieve paneer instead.
Another difference is that Greek yoghurt has much more protein - the kind I purchase has double the protein of regular yoghurt.
If you need Greek yoghurt for the thickness more than for the protein (making, for example, tzatsiki sauce), then you can strain it as bmargulies indicated.
I strain yoghurt in a coffee filter, over a coffee mug. Believe it or not, the damp filter clings to the mug enough that it doesn't just fall into the mug.