@Outis Oh well. Suppose I'll allaberate.
Fortune: Re-rolls failed saves, be they cover, invulnerable or armour. The Farseer blesses a squad or group with insane good luck essentially as bullets rarely find their mark.
Guide: Re-rolls to hit with shooting attacks. THe Farseer guids shots towards their targets with knowledge of where the targets shall be.
Doom: Re-rolls to wounds. Already explained.
Mind War: The Farseer smashes the enemies mind with her own physcic might which can obliterate a single opponent if they are undefended.
Eldritch Storm: The Farseer calls down a storm of physcic energy that throws squads around and damages them.
These are the powers mentioned in the codex but the books and rules mention so much more. Stopping time briefly, bonding individual threads of fate together so the death of one causes the others to die, Physcic Shrieks, Hallucinations, mental dominations, mental bolstering, runes of flame, farsight, misfortune. There are so many powers available. The only problem is that each use of a power can attract a daemon who may assualt the Farseer if her Ghosthelm doesn't keep her hidden. That and when you boil down to it she is an above average fighter at best who isn't physicaly strong or resilient.