I'm trying to make sure if Sharepoint is backup or if sharepoint DB is backed up, does it locks the DB and the end-user can't access the sharepoint. If end-user can access the site then whatecver the changes made during the DB are not stored in the backup?

According to http://technet.microsoft.com/en-us/library/cc706871.aspx , if you using build-in tools to backup SharePoint site collection, the site collection will not be automatically locked. If changes are made to the site collection during the backup process, the backup can become corrupted. Backing up large site collections can take a long time. To reduce the chance that user activity will interfere with a site collection backup, or that the time that is required to back up large site collections will exceed the available maintenance window, you should, for the duration of the backup, set the site collection URL to read-only by using the Setsitelock operation in the Stsadm command-line tool. This lets users view content on the site, but prevents activities such as adding or changing content that interfere with the backup process. When the backup is complete, return the access setting of the site collection URL to its default state. Further information is provided here http://blogs.msdn.com/krichie/archive/2006/07/25/678040.aspx .