Another one of those trendy fitness words that people like to use without understanding what they are talking about is Core (Functional training is the other: http://peak20workout.com/?q=functionalstrength).
A common fitness misconception is the abs are all that make up the core. Specifically, the part of the abs that you can see when you look in the mirror. Actually, the core is everything except the arms and legs (front and back of body). Thus crunches aren't the only way to train the core. Unfortunately, in the gym I hear trainers refer to the abs as to core. Don't get me wrong, the muscles of the abdominals are a big part of the core.

Furthermore, training the core is important for just about everything in life such as movement efficiency, athletic performance and for spinal protection. Train the core correctly and aesthetics will come as well. As in sports when you take a tumble you'll want to get back up with ease. Also a weak core leads to injury.This is just a short list of the many reasons to train the core properly.

Since all movement starts at the core it is a good idea to train the core at the beginning of a workout it also keeps the whole body activated during the workout.

Examples of core exercises:

Planks
Supermans
Medicine ball twists
Lateral bridge

Natasha Linton
Top Rising Star in Personal Training
-National Fitness Hall of Fame and Museum
Host of Peak 20 Workout TV
NYC Personal Trainer