Target-date funds make investing for retirement relatively easy. Choose a fund with the year in its name closest to the time you plan to retire, then sit back and let the fund's managers decide how much of your savings to invest in stocks and bonds (and occasionally other asset classes). But what happens when your target-date fund hits the target year?

Broadly speaking, there are two kinds of target-date funds: "through" and "to" funds. The glide path of one group takes you "through" the target date -- meaning it continues to shift the asset mix over a predetermined number of years. "To" funds stop adjusting asset allocations once they reach the target year. Whatever the track, most target funds eventually merge with a "retirement" or "income" fund with a stock-bond mix that stays unchanged.

Two popular target-date series, Fidelity Freedom and Vanguard Retirement, follow "through" glide paths, hitting the target year with stock allocations of 55 percent and 50 percent, respectively. Fidelity recently increased the duration of its post-target-date glide path from 15 years to 19 years, effectively keeping more in stocks longer. When a Freedom fund reaches its final allocation -- 24 percent in stocks and 76 percent in bonds -- it merges with Fidelity Freedom Income (symbol FFFAX). Vanguard's glide path continues for seven years after the target year, reaching a final mix of 30 percent in stocks and 70 percent in bonds. At that point, the target fund merges with Vanguard Target Retirement Income (VTINX).

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What about those "to" series, with allocations that stop gliding at the target year? The shifting may cease, but the funds don't die. For instance, two years after a target fund in JPMorgan's SmartRetirement series hits its target year, the fund merges into a static portfolio with 33 percent of assets in stocks, 50 percent in bonds and the rest in cash.

If you invest through a 401(k), you're limited to the series that your plan offers. But if you don't like a fund's mix, you can choose a fund with a different target year -- either further into the future or some years before you plan to retire.

(Nellie S. Huang is a senior associate editor at Kiplinger's Personal Finance magazine. Send your questions and comments to moneypower@kiplinger.com. And for more on this and similar money topics, visit Kiplinger.com.)