Core muscles are sometimes confused with the abdominal muscles. The rectus abdominis (abs) is part of the core which includes the muscles in your pelvis, lower back, hips and abdomen. Training your core leads to better balance and stability.
A bridge is a classic core exercise. Lie on your back with your knees bent. Keep your back in a neutral position, not arched and not pressed into the floor. Avoid tilting your hips. Tighten your abdominal muscles. Raise your hips off the floor until your hips are aligned with your knees and shoulders. Hold the position for as long as you can without breaking your form.
Heavy weight movement such as deadlifts and squats will train your core, and a strong core will in turn make you stronger in those lifts.
Check out this book to learn more about core functions and exercises: http://books.google.com.sa/books?id=7QVl60BQkRoC&pg=PA510&lpg=PA510&dq=core+stability+exercise+principles&source=vrt&ots=5YXzoTiJFk&sig=_C9nouuE9csqnITUZrqiONJ_1Fw&hl=en&ei=uT-OTc20CoaosAOHtPD4CA&sa=X&oi=book_result&ct=result&sqi=2&safe=on&redir_esc=y#v=onepage&q&f=true