What are Focus Items?

Are all necro,elementalist,monk, and mesmer items called Focus items?

Focus items = Caster off-hand items.

Basically anything that can't attack and isn't a shield.

and isnt a flute

I would say the flute is a focus of sorts - though a pointless one, unless it has some secret purpose - like a ceremony involving it, the tapestry shard, and the tattered girls cape, and several ritualists dancing around them, whilst a male necro shouts 'ATTINS, ATTINS, ATTINS!'.

But yes, a focus item is basically any energy giving 'off-hand' item.
They can have the following Attribute requirements:
Healing
Protection
Divine Favour
Curses
Blood
Death
Soul Reaping
Energy Storage
Fire
Water
Air
Earth
Domination
Illusion
Inspiration