Every Membership expires. When you are creating or updating a Membership you can setup Expiry options.

Day: expires same day

Calendar Week: expires end of that calendar week (end of Sunday)

Week: expires in 7 days

Calendar Month: expires at the end of that calendar month (if purchased in June, will expire last day of June).

Month: expires in 30 or 31 days (depends on the month purchased in)

Year: expires in 365 days

Calendar year: expires Dec 31st same year as purchased

Custom: set to expire a number of days, weeks or months after purchase date.

You can configure your membership to be a Subscription - a membership that renews at expiry, or you can set it to not automatically renew by Updating the membership and setting Allow Renew Subscription to Off.