1) simple, e.g.: and, but, so, though;
2) compound, e.g.: however, nevertheless, notwithstanding, etc.;
3) phrasal, e.g.: as soon as, on condition that, in order that, as if, as though, in case, etc.;
4) correlative, e.g.: both ... and, either ... or, neither ... nor, as ... as, etc.