I've seen zatar, za'atar, zahtar - all are transliterations from Arabic. I use zatar because it's just easy and phonetically straightforward, although I suppose za'atar is probably a bit more accurate (Orik? it is an ayin-ish thing?). I use it with fish a lot, also in a marinade I use for steaks and for chicken. Also make it on top of pita, mixed with olive oil (found all over the city in middle eastern places, by the way). I rarely use it alone - usually in a spice blend. Sometimes I make my own blend - but there are such good ones available in the middle eastern groceries, I usually buy it.