The crystalline lens is located just behind the iris.  Its purpose is to focus light onto the retina.  The nucleus, the innermost part of the lens, is surrounded by softer material called the cortex.  The lens is encased in a capsular-like bag and suspended within the eye by tiny "guy wires" called zonules.  

In young people, the lens changes shape to adjust for close or distance vision.  This is called accommodation.  With age, the lens gradually hardens, diminishing the ability to accommodate.