Mutual funds are a common investment choice for individuals to which, not too surprisingly, mutual funds make it a point to obfuscate the best choice for the investor, by offering more classes in regards to a mutual fund than is really necessary. For instance, some mutual funds, offer you the chance to invest in the same fund, but with different class distinctions of: A, B, C, R, and I. To the uninitiated, you might even think, that by giving you all these choices, that the said mutual fund is being consumer friendly, by making sure that you pick just the right fund for your needs, but in actuality, the long and short of it is that the mutual fund whether deliberately or implicitly or by just happy accident, is basically hoping that you slip up and select the fund that pays them a larger fee. If that wasn't the case, then the chosen mutual fund would reduce their class of funds to just very clear distinctions between one to another, with nothing in-between.
First, a brief description of each fund class type. Class A shares may have an "up-front" sales load that is paid up-front upon purchasing this mutual fund, but offers in return a lower annual expense fee, which will typically pay for itself in a very short of period of time, such as two years. Class B shares are virtually always the wrong choice as they have a higher annual expense fee, may also have a "back-loaded" fee, meaning that upon selling the fund that you pay a special fee, and to make matters worse they stick you with a perpetual marketing fee, 12b-1. Class C shares are similar to Class B shares, but do not have the "back-loaded" fee, and are also not convertible to Class A shares, to which, Class B shares may convert to Class A shares over an extended period of time. Class R shares are specifically built for retirement accounts, not so much because they are beneficial to the recipient, but more so that the mutual fund company can charge this particular class of shares at a higher annual expense fee. Class I shares, are for institutional investors or deep-pocketed individuals, that usually require a substantial investment amount to qualify, but these shares do not have 12b-1 fees, nor front or back-loaded fees, and additionally have the lowest annual expense fees.
Believe it or not, there are additional fund classes that are around for some funds but the above basically covers it all. Ideally, the best class of funds to invest in, is the I class, sometimes the threshold amount to qualify to invest is low enough that you will quality to do so, while also I class shares may be available through specific brokers with a lower minimum required. If you are unqualified to purchase I shares, you should as a given, never invest in B shares, and probably not invest in R shares, as the extra fee percentage is significantly higher than either A or C shares. Your default should probably be investing in A shares, but that is dependent on two things, whether there is an "up-front" sales load charge and how long you anticipate holding the shares. If you are unsure of the length of time you will hold your A shares or the "up-front" sales load is high, than the better part of valor is to stick with C shares.
It is up to you as in investor, to do the research on not just the fund itself, but the share classes that are offered. Most mutual funds will show you the past results of their different classes of their particular mutual fund, along with an explanation of the charges and expenses contained within. It is well worth your time to review this yourself before making your decision or, if in doubt, contact your brokerage company for further edification.