A vector layer represents geographic data whose features are given by sequences of points. (Contrast with a raster, which is a grid of values.)
Vector layers represent geographic features that are defined in terms of sequences of points (given explicitly in terms of their coordinates). This contrasts with raster layers which are grids of data (containing no explicit coordinates). Raster layers are usually used for aerial imagery and spatially extensive data such as elevations, while vector layers are used to indicate man-made features (such as boundaries and roads) and natural geographic features (such as coastlines, rivers, lakes, etc). Typically, vector layers associate data with geographic features in the form of attributes.