Main difference

As one who makes his living in the field of political polling/analysis/consulting, I think I have a fairly good idea of the definitional differences. A liberal almost always believes that a government which plays a more active role in society (via higher taxes, more rules/regulations) makes for a better society. By comparison, a conservative feels that individuals, left to their own devices, can much better run their own lives and therefore have a better overall society than with an overtaxing, overly regulatory government. With regards to political labels, Republicans tend (not always) to be conservative, whereas Democrats tend (not always) to be more liberal. This is overly simplified, but it's not a bad "working" primer.

"liberal" is a political standpoint which is often compared to "conservative." Democrats and Republicans are members of specific political parties which have both liberal and conservative standpoints on different issues.

"Republicans hire exterminators to kill their bugs; Democrats step on them. Democrats buy most of the books that have been banned somewhere; Republicans form censorship committes and read the books as a group. Democrats eat the fish they catch; Republicans hang theirs on the wall. Republicans tend to keep their shades drawn, althought there is seldom any reason why they should; Democrats ought to and don't"