In legal terms, "25 to life" means a prisoner is sentenced to a life term in prison but can become eligible for parole after serving 25 years. It is a way of establishing a clear minimum sentence without completely removing the ability for a parole release.
While the sentence may officially require 25 years before parole is available, in some cases this may be subject to prison population conditions, good behavior or other factors. If parole is not granted, however, the term may last the inmate's natural lifespan.
In the case of particularly heinous crimes, the sentence may forbid the possibility of parole and may just be referred to as a "life sentence."