PSU types and sizes?

[jassy_1]

Hi
I am planning on buying myself a new PSU as my old one is getting quite old. However, I am not exactly sure about sizes and things like that. Could someone maybe explain how it all works.
Thanks

 

[CountMike]

https://www.ibm.com/developerworks/library/pa-spec9/
https://www.google.rs/search?q=atx+power+supply+specifications&ie=utf-8&oe=utf-8&client=firefox-b&gws_rd=cr&ei=mUTzWLOoMIzRwALBypqQDA