A Plug_in normally come with an installer or install instructions. Check the documentation for the plug-in you trying to install. It sounds like you don't know what plug-ins you want to install. In that case you don't need to install any. At some point you may read about a plug-in and what it can do and feel you have a need for it. Then you will have a reason to install a plug-in that is not installed by default. There may also be a cost involved most worthwhile plug-ins are not free.
Adobe Optional Plug_ins downloads are Plugins Adobe want to remove from Photoshop and no longer install by default. Also at some point in time these will no longer install into the current Photoshop for Adobe will remove feature or interfaces the use. For example the CS5 optional plugins will install in Windows CS6 perpetual version 184.108.40.206 but not in Subscription version 13.1.2 or perpetual Mac version 13.0.6 fot its 64bit only and the Mac CS5 optional plug-in for Picture package is only a 32bit plug-in.
You will need to keep and maintain old versions of Photoshop if Adobe remove features you use in new versions of Photoshop.
Thanks , yes im still using adobe photoshop 7 when im trying to instal cs version its give error , and i already mentioned plugin n filters for special effect im new in adobe photoshop and few days before i was started adobe classes