Since ExpressionEngine stores most of your content in a database it is highly recommended that you perform regular database backups. Some hosting providers backup account databases as part of their service, but many do not. And even among those that do, not all of them perform backups at very frequent intervals so you are encouraged to make regular backups part of your site management strategy. Contact your host for more information.
PHPMyAdmin, available on most hosts, includes an export utility for creating database backups.