From Webster's dictionary:
Fund – a sum of money set aside for some particular purpose.
Pension – a payment, not wages, made regularly to a person (or his family) who have fulfilled certain conditions of service, reached a certain age.
Social Security – In the U.S., a federal system of old age, unemployment, or disability insurance for various categories of employment and dependent persons, financed by a fund maintained jointly by employees, employers, and the government.