Being non-American I don't know what ranch is (other than a place in which cowboys farm cattle teehee).  I assume it is some mayonnaise-based dressing which now that I look it up wikipedia appears to confirm but with a large amount of soured milk.  I don't think I have ever eaten it. I might like it, the seasoning described on wikipedia sounds interesting  Although I don't like mayonnaise which people say is weird because I like the British version called salad cream.