Invoker is a ranged intelligence hero capable of performing many different roles, but he is typically played a semi-carry spell-caster. He has 10 unique spells and 14 abilities, and so is only recommended for more experienced players.
Skills
Kill Build (4-1-4) - Get 4 in Quas and Exort asap to get two Forged Spirits. Get a value point in Wex early on. Max Exort First.
Key abilities are Cold Snap, Forge Spirit and Sun Strike.
Crowd Control Build (3-6-0) - Max Wex then Quas then Exort.
Key abilities are Tornado, EMP, Cold Snap.
Teamfight Damage Build (2-1-7) - Max Exort asap, then skill Quas and Wex according to taste.
Key abilities are Sun Strike, Chaos Meteor and Deafening Blast.
Items
Starting Items: either Blades of Attack, Healing Salve and a Branch OR Gauntlets of Strength, Circlet, three Branches and a Healing Salve or set of Tangos.
Early Core: Phase Boots, Bracer, Magic Wand. Hand of Midas if possible.
Mid-Game Core: Force Staff, Drums of Endurance.
Luxury Options: Aghanim's Scepter, Black King Bar, Scythe of Vyse, Eul's Scepter of Divinity, Orchid, Shiva's Guard, Refresher Orb.
Situational: Ghost Scepter, Blink Dagger, Necronomicon, Eye of Skadi.