When a mod refers to an Installation Method, they are referring to how a mod is installed. There are several methods of doing so.
Minecraft.jar[]
When a mod references this method, it means that all relevant files are to be directly added to the Minecraft.jar file, located in the /.minecraft/bin directory, using a suitable archive program, like WinRAR, or 7zip.
/mods[]
When a mod references this method, it means that the file(s) the mod uses must be dropped straight into the /.minecraft/mods directory, where they will be recognised by the game appropriately. Usaully, a /mods folder is generated if the player has installed Forge.
/coremods[]
When a mod references this method, it means that the file(s) the mod uses must be dropped straight into the /.minecraft/coremods directory. This is only used by a handful of mods. Usaully, a /coremods folder is generated if the player has installed Forge.
/.minecraft[]
This normally indicates that the mod comes as a package, and includes multiple items that must be installed in other directories within /.minecraft.