it depends if you are doing a warm (while database is online) or cold (while database is quiescent) bakup. If you are running onbar while online then yes, your database will be accessible but the backup will take much longer to complete depending the amount of users hitting the database (which will also be slowed down by the backup process)
Normally everyone is making online backups. The database is slowed down a bit but on most machines people doesn't really notice it. The only thing we see often is a somewhat larger checkpoint durection during backups.