A "cloud on the title" (according to my New York Real Estate Exam Review book) may be defined as "Any document, claim, unreleased lien or encumbrance that may impair the title to real property or make the title doubtful; usually revealed by a title search and removed by either a quitclaim deed or suit to quiet title." Bottom line, you don't want to "buy" property from a seller who can't unequivocally prove he or she has the legal right to sell it to you.
Sometimes it may be tempting, but it's always a bad idea.
Right after the "Velvet Revolution" in then-Czechoslovakia, in the spring of 1990, my Czech-born husband and I were in Prague where he was excitedly exploring all nooks and crannies of the beloved native city he had not seen since his hasty departure when the Russians sent their tanks rumbling through the streets in 1968. A posting in a cafe caught his eye- a "hunter's cottage" with land was available for $10,000! Unable to resist, we let ourselves be driven deep into the forest to view an archetypal whitewashed & thatch-roofed cottage offered for sale by a taciturn Russian officer soon to be on his way home. My husband was ready to have the money wired from America the next day; he was sure this bargain was going to be snapped up instantly if we faltered. But there was no easy mechanism for determining legal title at that time in the chaotic atmosphere of the disintegrating Soviet Empire; indeed, there were no reliable professionals handling private real estate transactions at all. For all we knew, the "seller" would disappear with his money and then the rightful owners would appear to claim property that had been confiscated from the Czechs long ago by their Russian occupiers. (Such restitution has happened very often in the years since, and rightfully so.)
So we passed on that opportunity, and waited 10 years before legally acquiring a small apartment in an Art Nouveau building on Korunni Street, near a streetcar line and small cafes.
I really did not want to live in an isolated cottage on the edge of a deep, dark woods anyway.
MORAL: You can take the New Yorker out of Manhattan, but you can't sell her a pig in a poke.