Meant to say, that in response to the original question, what law is, can probably best be described as a system. That system, of Courts, rules, and regulations, regulates behaviour within society, and punishes those who disregard those regulations. it also settles disputes by following those rules.
Lawyers are problem solvers. They solve clients legal problems by being able to identify issues, find the appropriate laws that deal with those issues, then apply it to the present facts. In order to do so they need to know how that 'system' operates.