As many others have said, in Microsoft SQL Server backups do not require a downtime. They automatically record changes that happen to the data while the backup is running. During the restore that change log is used to bring the database into a consistent state that lies somewhere between the start time and the end time of the backup.
If you need more control over the point in time to restore to, you need to also take log bakups. A database that is in FULL recovery mode allows you to take log backups. with a full backup and all log backups that where taken afterwards you can restore the database to any point in time after that full backup.
there are also differential backups that allow you to reduce the storage requirement for backups. To restore a differential backup you first need to restore the most recent full backup that was taken before the differential. Intermediate differentials are not required.
What type of backups to take and how frequent is dependent on you recovery point and recovery time objectives. (How long can the service be down? How much data loss can I afford?)
A starting point for planning you backup strategy is http://msdn.microsoft.com/en-us/library/ms191239(v=sql.105).aspx