Q: What are 'bandhas' and how are they used in yoga and yoga practice?

Mula Bandha (root lock) this is lifting the pelvic floor or perineum, the muscular tissue between the anus and genitals.

Uddiayana bandha (flying-up lock) this is the contraction from the anus to the diaphragm.

Jalandhara bandha (chin lock) this is when the spine is held tall and the chin is lowered to the crook of the neck.

The work "bandha" is an interesting one.  Farmers in the country in India take pieces of wood and stick them into the ground to help channel the moving water to different crops in different parts of the fields.  Those pieces of wood are called "bandhas".  In the same way that these farmers created water pathways and channels you can create energy channels and pathways in the whole body with the use of these locks "bandhas".  This allows a deliberate channeling of the energy and also a sifting out of impurities as the energy moves through the locks. Bandhas can be used in many postures and more advanced forms of pranayama (breathwork).  These are can be very beneficial in raising or balancing energy.  (I may add more to this one)

 Q:  When I am in forward fold I cannot reach my feet or toes?  Is it more important to reach my toes therefore bending my knees or should I not go in so deep?  How do I make sure I am doing the posture 'right'?

There is no truly 'right' way to do a posture.  The most important thing in yoga is that you are using awareness and attention and breath, this will ensure that you are safe in your body and avoid injuries while at the same time playing your edge--that place where challenge meets self-care.

In forward fold specifically the most important thing is that your breath is long and continuous.  If you force or crunch to achieve what you believe to be the 'right' position in a pose and lose the breath you have lost the most important element that will allow you to monitor your experience.  If on the other hand you continue to breathe deeply and begin with bent knees and use the breath to ease into the pose you will stay at attention, stay aware and stay connected--stay in yoga, while you are doing your forward fold. 

For forward fold, begin with bent knees and walk your hands down your legs slowly, elongating from the waist as you do so.  Use the strength of your arms to gently guide your chest out and over your legs, with knees bent if need be, breathing up an down the length of your back deeply, slowly and rhythmically.  If you have tight hamstrings or a hamstring injury, you can place a rolled up towel or blanket under the backs of your knees to prevent overstretching or damage to the hamstrings. As you breathe you will feel the body begin to open outward over the legs.  Do not stress your hamstrings but do keep moving forward until you feel that sweet place--the place where you are feeling your edge.  Your body is opening, responding to you and your breath, you are feeling interested in the posture and your body's response to it, you are present and aware and you are being conscious of your body so as not to 'push' or 'pull' yourself into a position that does not feel good just for the sake of reaching a pinnacle you believe you 'should' reach.

Yoga is a perfect mirror to see how we treat ourselves in our daily lives.  If we are pushing to achieve without honoring how we feel or our experience on the mat--chances are that we are doing the same in our work and personal relationships, our ambitions and the way we treat ourselves in general.  As we foster a practice of integrity and attention on our mats that will translate into our daily lives off of the mat.  That is living yoga and that is delicious.