Ostentatious comes from a Latin word meaning "display," and the idea of display is still very apparent in the English word as it is currently used.
People and things described as ostentatious seem to have put themselves on display; they are practically begging to be looked at. The word is not compliment.
Ostentatious is often applied to buildings that can also be described as luxurious—mansions, fancy high-rises, huge houses with marble columns. Sometimes the description appears in the negative, as when we're told that a house is large, but not ostentatious, which means that it's large but not in a way that calls attention to itself. When the word is applied to objects like clothes and jewelry, the idea is the same: such items attract attention for the luxury they imply.
People who are described as ostentatious—or who have lifestyles described as such—typically are seen as spending money in a way that makes it clear that they have a lot of it. Their consumption may also be described with the word, in which case the emphasis is on the impressive things they buy.
Less often, ostentatious is applied to what attracts attention not because of an implied luxury but because of some other quality. Someone's boastful declarations about volunteer work may be described, for example, as ostentatious.
