And to avoid someone quoting tracts from wikipedia, just read the damn thing here: http://en.wikipedia.org/wiki/Semantics
But, in summary, semantics is the study of meaning - without semantics, it is impossible to know the meaning of a word/statement/sentence/book, since all it consists of is a jumble of meaningless letters.
A certain “popular” poster on here seems to have his own brand of semantics and uses them liberally to infer bizarre or non-sequitur meanings from quite simple statements, whilst denouncing everyone else’s more conventional use of semantics as “just semantics”, “bullhockey”, and “holy guano, kiddo”.