Liquid cooling and water cooling are effectively the same thing. However, there are subtle differences in how each are viewed by the PC cooling community are a bit different:
Liquid cooling - often refers to smaller scale coolers, such as AIO (all-in-one) coolers that are self contained, liquid cooling loops. Think Corsair H-series, NZXT Kraken, etc.
Water cooling - typically refers to larger cooling loops that typically include a standalone pump, coolant reservoir, individual cooling blocks, radiators, tubing and fittings which are each individually selected and purchased separately, or as part of a kit.