There are essentially two kinds of mutual funds.
Open Ended Schemes: Investors can buy units of open ended schemes at any time. Investors can also sell units of open ended schemes at any time, though some schemes (e.g. equity linked savings schemes) may have a lock in period during which the investor cannot sell the units. The percentage ownership of investors in the assets of open ended schemes changes whenever investors purchase or sell units. Since you can sell units of open ended schemes at any time, high liquidity is ensured to the investors. However, costs may apply if you sell units of open ended scheme before a certain period of time from the date of investment. We will discuss this in more details later.
Close Ended Schemes: Close ended schemes are open for subscription only for a limited period of time, during the offer period. These schemes have fixed tenure and the investors can sell or redeem only after the maturity of the scheme. Upon maturity, depending on the scheme, the units get automatically redeemed or in some cases, the investors can switch to a different scheme. Close ended schemes are open for subscription only for a limited period of time, during the new fund offer (NFO) period. These schemes have fixed tenure and the investors can sell or redeem only after the maturity of the scheme. Upon maturity, depending on the scheme, the units get automatically redeemed or in some cases, the AMC gives option to unit holders to switch to a different scheme. The percentage ownership of investors in the assets of close ended schemes is unchanged throughout the tenure of the scheme as new investors cannot buy units post closure of the NFO. Some close ended schemes are listed on stock exchanges and you can buy or sell them through your share trading / demat account in the stock exchange, but the liquidity of these schemes listed on the exchanges is still quite low.