While this has become an everyday word in the fitness world, it is misleading.
First of all, your muscles either shrink (from inactivity) or grow (from resistance training). Decreasing overall body fat percentage will lead to more defined muscles, because your muscles will be more apparent when they have less fat covering them. This is not "toning"; you are just exposing the muscle that you already have.
In addition, your muscle shape is largely genetically-based. You're born with it. That doesn't mean there's no hope for you, it just means that doing 1,000 crunches everyday isn't going to give you differently SHAPED abs. That's why when some people have abs they are a "six-pack" and others have more of a hard, flat stomach with little cross-cutting. Either way, you won't see definition through your midsection unless the body fat that is there decreases.
Another really important clarification: your body doesn't spot reduce. Doing ab exercises everyday doesn't mean that your body is going to use that calorie deficit you put it in to reduce your abdominal body fat. Your body is (unfortunately) going to do whatever the hell it wants. Again, largely because of genetics. So even if you're doing more crunches than you are lunges, your body may lose fat from your legs before it does your abdomen...and for your workout buddy it could be the exact opposite! This is why it's so important to focus on overall muscle building and fat loss, not "toning".
Knowing this, it's best to focus on HOW to lose that body fat, build muscle, and uncover those shapely muscles you already have, or are currently working to develop.
8-12 reps is ideal for muscle growth (aka "hypertrophy"), although there are many different opinions on rep ranges. One thing fitness professionals do agree on, though, is that the high rep, low weight or even body weight workouts that claim they will help you "tone"...are BS. What is doing 25 crunches, 20 air squats, 15 burpees, and 10 pushups going to do for you? CARDIO, that's what.You may feel tired after completing one of these fad Pinterest-y workouts, but it's not going to do much for your muscle shape or size.
Generally, I do either 3 sets of 10 reps, 3 sets of 8, 4 sets of 8, or 4 sets of 6. This obviously varies depending on the exercises being performed.
If you're looking to decrease body fat, HIIT (High Intensity Interval Training) is a great way to go because it is a more intense, but shorter workout that utilizes both oxidative and anaerobic/ATP-PCr systems. (This is generally done in repeating intervals of 30 seconds high intensity and 1:30 of moderate intensity for about 20 minutes.)
THINK FAT LOSS, NOT WEIGHT LOSS!
A myth that goes hand in hand with this, is that women who lift weights will get "bulky" and look "manly". This is just. not. true. Seriously ladies...DO NOT BE AFRAID OF THE WEIGHT ROOM! Weights are going to give you those "toned" muscles, sexy curves, and ABS. Women just don't have the hormones necessary to bulk up like men do who lift unless they are eating a HUGE surplus of calories in order to purposefully gain that weight.
I did this in only 10 months by lifting heavy A$$ weights and barely doing any cardio. Just cleaned up my diet, and hit the weight room! I promise...weights are your friend.