The sfz format is an open, non-proprietry method of describing a set of samples and how a sample player should play them. It explicitly doesn't include the samples in the file, unlike Soundfonts or GigaSampler sets (but similarly to a number of other proprietry binary formats). It is very easy to create and - more importantly - easy to tweak.
SFZ files are just plain text, so they can be edited and viewed with an ordinary text editor. They consist of multiple <region> specifications which can be organised into <group>s. Each <region> has a number of opcodes specifying the circumstances under which the region is played.
Linuxsampler supports SFZ - see here for the list of supported opcodes.