NEW YORK ( WisdomTree) -- One of the many challenges Americans face today is saving enough money for retirement. The generations of Americans currently in retirement have predominantly received income from two sources: Social Security and pensions. However, since the early 1980s, the burden of funding future retirements has fallen on the individual. A significant decline in the number of pensions and an increase in the number of 401(k) plans have contributed to the current shortfall of retirement savings for many. Before we discuss the trend, let's take a look at what these two different types of plans are and how they work.A Defined Benefit (DB) Plan is a traditional pension plan in which the participant (employee) receives a benefit at retirement based on a predetermined calculation. An example of such a calculation would be receiving 75% of the average of the last five years' salary before retirement annually. If the average salary for the last five years before retirement is $40,000, then the pension benefit would be $30,000 annually in retirement for life. The employer usually makes the contribution into the pension plan for the participant based on actuarial calculations to fund the pension plan, and the employer controls the investment direction. A Defined Contribution (DC) Plan is a retirement plan commonly referred to as a 401(k) plan, in which participants (employees) contribute a portion of their salary into an account that provides a variety of investment options designed to meet the needs of the participant. At retirement, the participant still controls the investments and determines what is taken out of the account as income. A 25-year-old who put $2,000 per year into his/her 401(k) plan could expect to have an account balance of $374,000 at age 65, assuming his/her account grew at an average annual rate of 6.5%. The participant makes the contributions into his/her account, with the employer in some plans making a matching or profit-sharing contribution to the participant's account. In most cases, the participant controls the investment direction. According to the Department of Labor, the number of private pension plans in the U.S. has decreased from a peak of 175,143 in 1983 to a low of 46,543 in 2010. During the same time period, the number of profit-sharing and 401(k)-type retirement plans has grown from 207,748 in 1975 to 654,469 in 2010. Even more compelling is the trend with private pensions and 401(k) plans with more than 100 participants. Those private pension plans decreased from a peak of 25,979 in 1983 to a low of 10,155 in 2010. The number of 401(k) plans grew from 17,303 in 1984 to 518,675 in 2010.