Universal detector means it is useful to detect the most of the compounds like refractive index detector based on their physical property. Bulk property detector means not specific to solute but sometimes its useful to solvent detection.
ELSD and RI detection are the usual ones. They are most often used with molecules like carboydrates that do not have a chromophore (cannot see by UV) and do not ionize well (cannot see by MS).
ELSD is nice because, unlike UV and MS, the response is the same for all molecules and is proportional to mass. It shows the "truth" of the relative amounts of different components in a sample. But its not as sensitive.
Varun correctly answered your intial question as "bulk property" detector do indeed detect samples based on a physical characteristic.UV/VIS and RID are good examples.
ELSD response is NOT the same for all molecules and is not proportional to mass (that was a sales slogan used to sell the detectors and based on some initial theoretical work). Their actual response is different for different compounds and varies depending on the mobile phase, detector settings chosen and range calibrated across.
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