The goal of the Lensfun library is to provide a open source database of photographic lenses and their characteristics. In the past there was a effort in this direction (see http://www.epaperpress.com/ptlens/), but then author decided to take the commercial route and the database froze at the last public stage. This database was used as the basement on which Lensfun database grew, thanks to PTLens author which gave his permission for this, while the code was totally rewritten from scratch (and the database was converted to a totally new, XML-based format).
The Lensfun library not only provides a way to read the database and search for specific things in it, but also provides a set of algorithms for correcting images based on detailed knowledge of lens properties and calibration data. Right now Lensfun is designed to correct distortion, transversal (also known as lateral) chromatic aberrations, and vignetting. Furthermore, it can convert between different types of lenses, for example from fisheye into rectilinear.