What is the Gabor filter? And what are its main uses?
Gabor filters are orientation-sensitive filters, used for texture analysis.

The typically travel in packs, one for each direction. A gabor filter set with a given direction gives a strong response for locations of the target images that have structures in this given direction. For instance, if your target image is made of a periodic grating in a diagonal direction, a gabor filter set will give you a strong response only if its direction matches the one of the grating.  

I know it is used a lot in character recognition and fingerprint enhancement. I (try to)  use it in bio-medical imaging to characterizes the main orientation in fibril structures. 

Here is a very good tutorial by Javier Movellan, pdf

And if you can read french, here is pdf on the creation of filter banks by Adrien Marion