Well.. this hero is insanely flexible. He can be played as some combination of nuker/disabler/pusher. Personally, I think his disables (especially ice wall, emp and deafening blast) are his most powerful asset, but it's crucial to be flexible and experienced enough to use all of your spells.
I highly suggest you read more about Invoker than this.
Build 1 for items
Summoner Abilities: Forge Spirits, Ice Wall, Cold Snap
Disable Abilities: EMP, Tornado, Blast
Nuker DPS Abilities: Alacrity, Meteor, Sun Strike, Blast