What it does
This program builds a VRT (Virtual Dataset) that is a mosaic of the list of input GDAL datasets.
If one GDAL dataset is made of several subdatasets and has 0 raster bands, all the subdatasets will be added to the VRT rather than the dataset itself.
gdalbuildvrt does some amount of checks to assure that all files that will be put in the resulting VRT have similar characteristics : number of bands, projection, color interpretation... If not, files that do not match the common characteristics will be skipped. (This is only true in the default mode, and not when using the -separate option)
If there is some amount of spatial overlapping between files, the order of files appearing in the list of source matter: files that are listed at the end are the ones from which the content will be fetched. Note that nodata will be taken into account to potentially fetch data from less prioritary datasets, but currently, alpha channel is not taken into account to do alpha compositing (so a source with alpha=0 appearing on top of another source will override is content). This might be changed in later versions.
This utility is somehow equivalent to the gdal_vrtmerge.py utility and is build by default in GDAL 1.6.1.
How to use it
Add as much input files you want to use to create the VRT.
Advanced options and sources
To see complete details and help section please check the official gdal sources.
http://www.gdal.org/gdalbuildvrt.html
Raster Processing Tutorial : https://trac.osgeo.org/gdal/wiki/UserDocs/RasterProcTutorial