There are several differences.
First, An early retirement can only be taken when one has been offered by the employing agency, usually because there is a major reorganiztion, a reduction-in-force, or a transfer of function.
When an early retirement is offered, the “eligibility rules” are different:
– age 50 and 20 years service or
– any age with 25 years service.
If the retiree is under age 55, there is also a reduction in the annuity.
For an optional retirement (meaning the employee chooses to retire, under normal circumstances) the eligibility requirements include:
– age 62 and 5 years, or
– age 60 and 20, or
– age 55 and 30.
Check this site: http://www.opm.gov/retire/faq/pre/faq11.asp