The first can be viewed as a hierarchical memory model, where you store a texture for the entire environment at relatively low resolution but provide high-resolution "insets" in regions in which the viewer might want to zoom in. This was done quite successfully for flight simulators, where you might have the entire environment stored at low-res but then a few buildings are stored at high resolution in another part of memory. When the user zooms in to these regions, the renderer automatically indexes the high-resolution textures to provide the extra detail.
The other approach also known as detail maps involves a low resolution texture which covers the entire environment and is then modulated by a high resolution tileable texture. Because the high-res texture can be tiled, it does not take up too much texture memory, yet it allows the user to move up close and still see sharp detail. An example of the high-res tile is shown here in gray and it is applied to the terrain shown here to get this result.