Within the setup of a Membership Level, 5 individual settings relate to the Expiration of a Level. You can allow for a monthly membership or set a membership to expire a certain number of days from the date of purchase. You can also set the expiration to move itself to the end of the month that the expiration would be in. Or you can set up a membership that is for a Lifetime and has no expiration date. All of these four settings fall under what we call a Rolling Membership. This is because the Member can continue to purchase and renew this same Membership Level repeatedly and the expiration date will always set be in the future.
There is also an option for a hard date expiration. To do this, select a specific date on whichever Membership under that Level will expire. This is different from a Rolling Membership because this level cannot be renewed. A new Membership Level would need to be created each time you have a new membership period and the old level inactivated.