Those mutation descriptions are called HGVS (Human Genome Variation Society) nomenclature. In the example you give, "c." means that it's in a (protein) coding region, 345 is the position within the region, and G>E would be the change (although E isn't a valid "letter" in DNA sequence, even if you allow ambiguity codes -- you'd normally see something like G>T there instead).

Complications include:

1) You need to know which gene this is relative to.

2) The "coding sequence" for the gene isn't always perfectly defined, due to splice variation and different versions of the annotation. Ideally, you'd see this code relative to a specific splice variant (which might have an ENST identifier, from http://www.ensembl.org/). But it depends...

More at http://varnomen.hgvs.org/ if you're curious.