I may be entirely wrong but when i think of fasting, i think of not eating at all for the day. Obviously this may not be healthy for some people, so eating little throughout the day, maybe one meal, is the alternative.
And i guess being in the younger generation, the first thing i would think of when hearing abstinence is from sex. It might be logical to abstain from sex to give your entire devotion to God for that day. But then again we do abstain from meat for sure… so maybe both? =D
I may be entirely mistaken as well, so if anyone is confident in their answer, please share! =D