CPF Life is the alternative to the CPF Minimum Sum Scheme. The CPF Minimum Sum Scheme provides monthly payouts for about 20 years whereas the CPF Life Scheme gives a lifelong monthly payment when you turn 55. How much your monthly payout depends on the plan you are on. There are a few plans currently but next year (2013) onwards, the plans will be simplified to 2 - basic and standard.

For people born 1954 or earlier - you can choose to join CPF Life or remain on the Minimum Sum Scheme.
For people born 1955 to 1957 - you can apply to join CPF Life when you reach 55 years old or remain on the Minimum Sum Scheme.
For people born 1958 or later - you will be automatically included under CPF Life if you have at least $40,000 in your Retirement Account (RA) at 55 or at least $60,000 in your RA at 65. If you are not automatically included, you can still apply to join the CPF Life Scheme.