libecwj2 is a library that gives support to GDAL for reading, import,
and convert raster geospatial data in non-free ECW and JPEG 2000
formats. This library was released as free and open source in 2006 by
Earth Resource Mapping, formerly owner of ECW patents.

ECW (Enhanced Compression Wavelet) is a full proprietary standard of
compression image format. 
JPEG 2000 (or JP2) is a similar standard under free concession but 
protected by patents in some parts.

Both the formats are more powerful than other formats in lossy 
compression of large image data without significant losses of quality,
but are mainly accessible through proprietary software, specially to
manage geospatial data.

libecwj2 is the only one free and open source library that includes
the support to these formats in GDAL and indirectly in platforms that
require it as dependency (i.e. QGIS and Orfeo Toolbox). GDAL can be
also used to convert ECW or JP2 files in other graphic formats.

After the installation, to apply the support to ECW/JP2 formats, GDAL
must be rebuilt without any other setting: libecwj2 is an optional
dependency autodetected.

To verify if the plugin is enabled, you can launch the following
command:

   $ gdalinfo --formats | grep ECW
   
If the plugin is successfully enabled, this command returns

  ECW -raster- (rw): ERDAS Compressed Wavelets (SDK 3.x)
  JP2ECW -raster,vector- (rw+v): ERDAS JPEG2000 (SDK 3.x)