[Baron Spikey]

well there's only 8 Careers but each one branches off into 2 or more specialities and obviously the skills, talents and other customisable things alter your character even further. I'll give a basic description of each of the final Ranks for the Careers.

Adept
Loremaster Magister:

"When you understand the secrets of the universe, you will understand yourself"
Loremaster have reached such a level of knowledge of the galaxy that they have now turned their attentions inwards, looking into the very mysteries of the human soul.

Sage Logister:

"They say even the mysteries of the Tyrant Star are not beyond the mind of a sage"
Few secrets remain hidden from the mind of a sage.

Arbitrator
Lord Marshal:

"Such men are beyond mere mercy"
As the Imperium is mighty, so too is the Lord Marshal. His will is not blunted by the petty whims of circumstance. The cold rule of law is his only measure, and woe betide those who fall short.

Justicar:

"Where Justicar tread, nobles tremble"
The Justicar brings the light of the Emperor's judgement to decadent nobles, corrupt officials and worlds brought low by dark worship.

Assassin
Imperator-Mortis:

"I am death incarnate"
The Imperator-Mortis is an embodiment of graceful slaughter, an angel of death with few peers.

Assassin Palatine:

"He is the lord, the master, the very god of assassins. When he says kill, you'd damn well better"
Where there are assassins, so too is there the Assassin Palatine. Often in control of vast guilds of killers, Assassins Palatine are both charismatic leaders and cold-blooded killers.

Cleric
Hierophant:

"Of course this is a matter for the Ecclesiarchy. Why? Because it involves the Imperium of course..."
The Hierophant whispers in the ears of governors and kings, counselling on matters spiritual, political and malefic. Wise, yet also deadly, Hierophants study the greatest enemies of mankind, to discover how they may be crushed beneath the armies of the nations they advise.

Redemptionist:

"Burn the heretic!"
The Redemptionist leads the battle charge, spewing litany and destruction upon those that would seek to harm the Emperor, the Imperium and man.

Guardsman
Storm Trooper:

"Storm troopers? Them throne-groaning glory boys get all the prime action- and all the credit, too. Don't listen to the propaganda- it's us regular Guard that does all the grit-work"
Storm Troopers create utter destruction. Often regarded by the regular Guard as vain parade apes, they nonetheless have a well-deserved reputation for being the best of the best.

Commander:

"Men of the Imperium stand tall, for you are the favoured of the Emperor!"
Commanders control the entire flow of battle, from planning to execution. Imperial Commander have at their disposal the greatest armies in the known galaxy, and the cold, hard determination to use them.

Sniper:

"There's something really satisfying about making a man's head explode like a blood melon. All in the name of the Emperor of course."
Snipers are masters of their art, able to kill at distance, then melt away from view. They are silent, skilled and often have a grim sense of humour. Many find them hard to understand.

Imperial Psyker
Preceptor-Savant:

"You know how Commissars are strict? Well that ain't nothing to a Preceptor. Shoot his men as soon as blink- and a good job too."
The Preceptor-Savant is a psyker charged with manifold duties- to lead the Savant Militant, to protect key command staff and to instruct officers in the best deployment of psychic forces.

Scholar Empyrean:

"Sire, success! He made three pounds of gold yesterday! Funny fish though, didn't look happy or nothing, just wanted to be left to his proper studies....whatever that means."
The mind of the Scholar Empyrean ranges free, even whilst their will is securely chained to the immeasurable protection of the Emperor's own soul. Studying the deepest mysteries of psychic ability, the Scholar Empyrean care little for worldly goods or the petty demands of the corpus.

Scum
Gang Lord:

"The Robber-Monk they calls him, sleek as silver, twice as quick. Could steal your shadow whilst you're blinking, so they say. The Spire nobs are quaking in their boots. You mark my muzzle, there's more than one family safe that'll be cracked tonight!"
The Gang Lord is skilled in high-stakes operations, from assassination to cat burglary and data-heists. He can put together a crew, or simply awe the local underworld with his low cunning and high returns.

Charlatan:

"He seemed like such a nice young man..."
The Charlatan is a master thief, who can bind even planetary governors to their web of elaborate deceit. Using sophisticated and long-term plans, they are architects of cunning, subterfuges that put even the sharpest of lawmen on the back foot.

Tech-Priest
Magos:

"An Adeptus Mechanicus Magos is a creature of narrow, but exceedingly deep vision. Each chooses a realm of dominion and sets about learning all there is to know about said subject. Be wary of them, of their cunning and their obsessions."
A Magos has perfected and refined his field of expertise to render him the master of a certain study. From the fleshwise magos biologis to the alien hunting magos xenologis, these sage individuals serve the Adeptus Mechanicus with their enormous accumulated knowledge.

Magos Errant:

"What we know of the Adranti Blood-Lighters is from the work of Abnightus, a magos errant attached to the Rogue Trader vessel, Luminol. The scroll is expunged of Adeptus Mechanicus matters, but still, we can infer much from what remains."
The master of many trades, the Magos Errant is expected to to go forth on behalf of the Adeptus Mechanicus, to explore new frontiers, worlds and technologies. Privy to all manner of secret knowledge, the Magos Errant is prepared for all eventualities.

EDIT: by the way I'm aiming to get my character to follow the career path that eventually leads to the Storm Trooper