The length of a "life" sentence depends on the crime being punished and the state in which it was committed. While a life sentence may actually send inmates to prison for life, guidelines often allow inmates to be eligible for parole after serving a specified period of their sentence.
Persons sentenced to 10 or 25 years to life are eligible for parole after 10 or 25 years, respectively. After serving the specified minimum, the inmate may appear before a parole board who decides whether or not to grant the early release based on behavior in prison, perceived threat to society and other factors.
In comparison, a "natural" life sentence means the defendant is to spend the rest of his natural life in a prison cell, with no chance at parole.