Shakespeare put these words in the mouth of Julius Caesar: "Yond Cassius has a lean and hungry look; He thinks too much: such men are dangerous." Cassius' envy and his fear of Caesar's growing power and of Caesar's great ambition led him to persuade Brutus that Caesar had to be stopped. (The Brutus story is at http://www.mmdtkw.org/VBrutus.html.) Lean and hungry Cassius was, greedy for ever greater power and wealth.
Shakespeare was close, but Plutarch, who recorded Caesar's words almost fifteen hundred years closer to the event, recorded it thus "It is not, the fat and the long-haired men that I fear, but the pale and the lean." Similar words with essentially the same meaning, and Shakespeare's scanned better. Both meant that Caesar fully understood the ambition and greed of Cassius.
Who was this Cassius? Of a noble Roman family already famous for its civil and military services to Rome, his own early life was either not been recorded or lost. He first appears in the literature in 53 BC as one of the commanders in the army of Marcus Crassus at Crassus disastrous defeat by the Parthians (ex-Seleucids) at the Battle of Carrhae (Haran) in Mesopotamia.
There has always been some question about Cassius' actions at Carrhae: his partisans said that Cassius had seen that Crassus was already defeated and therefore declined to throw away the lives of more Roman troops; his detractors said that he stood by, keeping his forces out of the battle, and let Crassus go down to ignominious defeat, capture, and execution; conspiracy theorists guessed that he had accepted promises of future preference and held back to let the Parthians clear Crassus from the path of Pompey -- or of Caesar. Whatever the circumstances, Cassius reorganized the Roman remnant that escaped at Carrhae, arranged for their augmentation under his own command, and won a minor victory against the Parthians the next year.
Cassius then established a power base in Syria that allowed him to extort money from anyone who wished to trade in his area, and this enabled him to increase his wealth significantly. Cassius was appointed Tribune in 49 BC. He sided with Pompey and the corrupt "republican" Senate against Julius Caesar, and he was Pompey's naval commander off Sicily in the civil war that ensued. Cassius was still on Pompey's side when Pompey was routed at the battle of Pharsalus in Thessaly (Greece) in 48 BC, but, shortly after Ptolemy delivered Pompey's head, Caesar forgave Cassius and tried to co-opt him by making him a legate.
After Caesar's pardon, which also extended to many others among Pompey's former allies, Cassius quickly slipped back into the opposition and became deeply involved in "republican" causes in Rome. That really meant that he conspired with other corrupt senators, who claimed to want a return to the republic but whose real goal was to thwart the mercantile, monetary, and civil/military service reforms proposed by Julius Caesar. Those reforms were to be implemented by Caesar's governor in Rome, Marc Antony, while Caesar was chasing Cleopatra in Egypt. Meanwhile Cassius was wooing and marrying Junia, the half sister of Brutus, another pardoned Pompey partisan and "republican" activist. (If this sounds really complicated, it's only because it really was really complicated.)
Probably even before Caesar returned to Rome in the fall of 45 BC with Cleopatra on his arm and a plan to end the Senate's corrupt system of military and civil preferments in his pocket, a plot had been hatched to assassinate Caesar at the first opportunity. Cassius was one of several leaders of the plot and, after the fact, it was decided that he had been the key plotter.
About 60 senators were directly involved, and the standard account of the assassination of Julius Caesar says that twenty-seven of them managed to stab Caesar with their swords and daggers when he stopped to receive a petition at the foot of the statue of his old enemy Pompey at the temporary Curial meeting hall behind Pompey's theater. This happened only six months after Ceasar returned to Rome, but in that time he'd made enough stupid mistakes to infuriate all of Rome's classes and factions. His ineptitude was particularly obvious when his admiinistration was compared with that of Marc Antony, who had ruled in Caesar's absence. Nobody really knows if twenty-seven blows were actually struck or if the number 27 had some numerological, tribal, or political significance. No matter: Caesar was dead in an initially popular assassination.
But Marc Antony, knowing that his own political future depended on casting the dead dictator as a popular military hero, quickly orchestrated a public outcry against the assassins. Caesar was burned on a makeshift warrior's pyre in the Forum, and Cassius and the other conspirators were forced to flee Rome.
Cassius eventually went back to his old power base in Syria, and there after defeating Antony's governor, Dolabella, and taking possession of the ill-fated horse, he raised a big army from the legions that were loyal to him personally. In 42 BC, he joined forces with his brother-in-law and co-assassin, Brutus, and their combined armies waited for the legions of Marc Antony and Octavian (later Caesar Augustus) at Philippi. The battle on the field was essentially a draw: Antony's forces broke Cassius' army and entered his camp, but Brutus had defeated Octavian and was coming to Cassius' rescue. Cassius, seeing only the smaller picture, the enemy troops in his camp, and not knowing that salvation was at hand, ordered his trusted shield bearer to help him commit suicide. According to legend, the soldier dealt the death blow with the same sword that Cassius had used in the assassination of Caesar. When word came of the suicide of Cassius, Brutus also despaired and joined Cassius in suicide.
Thus ended the last of the "liberatori" who had slain Caesar, ostensibly to restore the republic but actually to retain their corrupt privileges. History liked Brutus more than Cassius who took most of the blame for the plot. In fact, there were no good-guys here. All, including Caesar, were in the game for what they could win by whatever means. They all died and their heirs sorted out what was left and spun the histories the way they wanted to.
A brief Cassius timeline: http://ancienthistory.about.com/homework/ancienthistory/library/bl/bl_cassius.htm
Britannica on Cassius: http://www.britannica.com/eb/article?eu=20994
Princeton Economics on Cassius: http://www.princetoneconomics.com/history/ROMAN/Cassius/Cassius.htm
Plutarch -- life of Brutus (Search text for Cassius and the assassination of Caesar): http://www.anx.liverpool.k12.ny.us/staff/Anello/tt/reading/brutus.html
Shakespeare and the assassination of Caesar: http://www.perseus.tufts.edu/JC/JC.shak.home.html
1. Cassius had the unusual distinction of being on the loosing side at three of the major battles of his time: Carrhae, Pharsalus, and Philippi. Had he not despaired at Philippi, he may also have gone to Actium.
2: Another Gaius Cassius Longinus, a direct descendant of the famed assassin, appears as an author, jurist, philosopher, and enemy of Caligula and Nero (and therefore a good-guy) in the mid-first century AD. Nero had him banished, but Vespasian rehabilitated him and brought him back to Rome for an old age of public adulation.