The topic seems to be pretty well covered already, but as a general guideline:
high reps, low intensity = fat burn and toning
low reps, high weight = muscle building
the actualy proportions are very individual. I find i get good toning of hte muscle by just doing heavy weights a medium numeber of times (10 - 15). But optimal growth is at 6-8 reps.
Lower abs are the hardest muscle to build on the stomach. As mik1w said, leg raises are good. Also, lying down on your back, and jsut holding your your feet a foot off the ground with legs straight is good. There are lots of variations, I use a medicin ball between my feet for leg raises, so there is a little extra weight.
For arms different people like different things. I prefer to isolate muscles, while others prefer compound excercises. Try and see what works for you.
P.S. I know everyone says it, but it really is individual, it took me a few months to work out which excercises i liked. As some really dont do anything for me.