Contrary to popular belief, this is NOT some kind of STD (that would be a UTI, not a UIT). A UIT, or Unit Investment Trust, IS actually a very popular investment tool, not too much unlike an Exchange Traded Fund (ETF) or a general mutual fund. While not one of the most popular investment instruments out there, they are still a fascinating investing tool to learn about and potentially considering adding to your retirement account.

With a little help from Investopedia, here's what you need to know about UITs. For subject matter related to UTIs, consult your doctor or WebMD.

What is a 'UIT'?

A unit investment trust (UIT) is an investment company that offers a fixed portfolio, generally of stocks and bonds, as redeemable units to investors for a specific period of time. It is designed to provide capital appreciation and/or dividend income. Unit investment trusts, along with mutual funds and closed-end funds, are defined as investment companies.

How are 'UITs' sold?

Investors can redeem mutual fund shares or UIT units at net asset value (NAV) to the fund or trust either directly or with the help of an investment advisor. NAV is defined as the total value of the portfolio divided by the number of shares or units outstanding and the NAV is calculated each business day. On the other hand, closed-end funds are not redeemable and are sold in the secondary market at the current market price. The market price of a closed-end fund is based on investor demand and not as a calculation of net asset value.

So what is the difference between a 'UIT' and a 'Mutual Fund'?

Mutual funds are open-ended funds, meaning that the portfolio manager can buy and sell securities in the portfolio. The investment objective of each mutual fund is to outperform a particular benchmark, and the portfolio manager trades securities to meet that objective. A stock mutual fund, for example, may have an objective to outperform the Standard & Poor’s 500 index of large-cap stocks.

Many investors prefer to use mutual funds for stock investing so that the portfolio can be traded. If an investor is interested in buying and holding a portfolio of bonds and earning interest, that individual may purchase a UIT or closed-end fund with a fixed portfolio. A UIT, for example, pays the interest income on the bonds and holds the portfolio until a specific end date when the bonds are sold and the principal amount is returned to the owners. A bond investor can own a diversified portfolio of bonds in a UIT, rather than manage interest payments and bond redemptions in a personal brokerage account.