Difference - Site Backup/Restore and Export/Import

Site Export and Import is used for Content Migration. The Content Migration API's are designed to migrate content from one source location to a destination location - they are not designed for backup and restore. Content Migration API has following limitations:
1. No Configuration or Application data can be exported or imported.
2. The larges object you can export is WSS web site (spWeb object)
3. Following Contents cannot be exported or imported - alerts, audit trial,change log history, checkin/checkout state, recycle bin items, recycle bin state, security state, workflow state and workflow tasks
4.Although web parts are migrated between source and destination sites, no assemblies are migrated, if you have custom web parts then you have to manually install the assemblies in the destination location

There are 3 ways to invoke content migration API's
1. using STSADM utility - you can use import and export operations to migrate data. (limited to website only)
2. you can use ExportWeb or ImportWeb methods implemented in the sites web service
3. Content Migration Object Model (Microsoft.SharePoint.Deploymen) - you can migrate anything from a website to item in a list, or a single document in a library. You can choose whether to include information about security, versioning, user roles and other metadata of the objects that you are migrating