Single pass scanners are preferable because they are able to scan quicker and may have fewer registration problems when the scanner has been used for a long time. Twain is a software interface standard developed by Hewlett-Packard for interfacing scanners to productivity software. The general idea is that a device with a twain driver can be used with any software that supports the Twain standard. This allows someone to operate a scanner directly while using Photoshop, with the resulting scan showing up in Photoshop's edit buffer. Twain support is mainly used in the Windows environment.
Many manufacturers write their scanner software to work as a plugin.
This arrangement is often used on the Macintosh, but graphic packages for Windows are supporting the Adobe plugin standard as well. This is an optional lid for flatbed scanners that has a light source inside, usually florescence. It is useful for scanning transparencies, x-rays, etc. It can be used for scanning slides as well, but a slide scanner will do a vastly better job.