In Transformers mythology, the idea of the spark is something not easily explained. We were first introduced to the concept in Beast Wars, but earlier nods to it were present in the original series as well. Of course, the writers of the 1984-era cartoon had no idea that the premise would be resurrected more than a decade later, but they understood then as we do now that there was some essence in all Transformers that gave them their individual personalities but also that made them more than mere machines. In the original series, the term "laser core" was thrown around every now and then and it seemed as if this was the biological (or perhaps mechological) heart of a Transformer. However, we also learned later on in a bit of retcon that only the Vector Sigma super computer was capable of bestowing life upon Transformers. (How the Dinobots and Constructicons were given life is never explained, given that they were built on Earth before the writers came up with the Vector Sigma plot device.)
Fast-forward to Beast Wars. We learn here that the spark is a lot of things. It is simultaneously a Transformer's mind, heart, soul, and lifeforce. A Transformer dies when its spark leaves its body and enters the Matrix. However, something that isn't entirely clear is what happens after a spark is extinguished. Does the spark join the Matrix in the physical sense or purely metaphysically? Does it fly into Optimus Prime's chest or does it ascend to some higher state of being? Is it just a religious lie? In Beast Machines, Blackarachnia's spark left her body and she entered into a state of limbo with Optimus Primal, whose spark was still within his own body, but he was meditating. When Optimus Primal was in limbo, he nearly entered the Matrix, but was returned to life after he completed his mental "training."
Likewise, to what extent does the spark determine personality? Looking at the original Vehicon generals is the first clue. Jetstorm, Thrust, and Tankor possessed the sparks of Silverbolt, Waspinator, and Rhinox, respectively. Let's examine them.
In Beast Wars, Silverbolt was defined as a noble, chivalrous character. He was very much like our idealized version of what a knight should be; he was stoic, respectful, a good team-player, and prepossessed by the idea of honor. Now, let's take a look at Jetstorm. Jetstorm was cocky, vain, arrogant, and had little concern for anyone other than himself. There were two exceptions to this: Megatron, since he could elevate his status within the Vehicon ranks; and Thrust, since they were kinda-sorta friends. Jetstorm was not afraid to lie, fight dirty, or attack his own teammates (as was the case with Tankor). When he believed Tankor to be dead, he said to Megatron "your glorious army will shine less brightly without his guiding! Can I have his tank drones?" That is the summation of Jetstorm's character. He pays lip service to those in a position to advance himself (Megatron). The two characters seem to be diametrically opposed except for one common point of origin: dedication. Both Silverbolt and Jetstorm are dedicated to their respective causes. Megatron is said to have "corrupted" Silverbolt's spark, but that's not really a fair way to describe what has happened. He took Silverbolt's spark and put it into a different context; Jetstorm is the polar opposite of Silverbolt, but the two share a common underlying ideal: dedication to causes. How their personalities differ from there is simply a literary point: a bit of poetic injustice, if you will. This idea of personality inversion is seen within the other two generals as well.
Waspinator quickly established himself as something of a fan favorite; he was comic relief in Beast Wars. He was slow-witted, spastic, had an annoying voice, and was extremely "uncool." Thrust, on the other hand, was observant, calculating, laid back, had a great voice, and was extremely cool. Put simply, he was the inversion of Waspinator. How he differs from Jetstorm, however, is that his point of origin is his loyalty to Megatron rather than to a particular cause. Waspinator was with Megatron from the beginning; even before they left Cybertron with the Golden Disk, he was in his group. Megatron recognized that personal loyalty and built a sleek, cool general around it. Thrust is seen denouncing the Maximals, Silverbolt, Obsidian, and Strika throughout the series for their lack of loyalty. He says to the latter two "if you're loyal to everybody, can you really be loyal to anybody?" This is important because Obsidian and Strika were designed to protect Cybertron in whatever form it exists. Since Megatron controlled Cybertron, they were loyal to Megatron. When Megatron was believed to be dead, they weighed the option of changing alliances, much to Thrust's anger. Thrust is thereby not driven by a cause, but rather by a personality. When Megatron compares him unfavorably to Jetstorm and Tankor after a failed mission, Thrust retorts "at least I didn't turn on you like they did." He elevates Megatron above any cause. Whereas Obsidian and Strika would be loyal to Cybertron no matter who was in charge, Thrust would be loyal to Megatron no matter what his plans were.
Probably the most controversial of the three generals was Tankor. As the Maximal second-in-command Rhinox, he was a gentle giant. He was generally soft-spoken, respected the natural, and was a fatherly figure to Airrazor. He was a true friend to Optimus Primal. He was a genius; he designed most of the Maximals' weapons and was their "techie." As Tankor, he was a nearly mindless brute. He spoke in extremely short sentences and was quite misanthropic. But he was strong. Physically, he was the strongest character of the first season of Beast Machines. His strength was the point along which Megatron inverted him. Gone were his love for nature, his kindness, and his intelligence. Eventually, Tankor regained his Rhinox personality...partially. When Optimus Primal finally broke through, Rhinox turned him away. He refused to be called by that name. He considered Primal and the Maximal cause to be failures. He claimed that Megatron was right about Cybertron but went one step further: he claimed that he was the one who should rule a technological utopia rather than Megatron. Rhinox became cruel, manipulative, and on-the-whole evil. Optimus Primal briefly considered reformatting Tankor into Rhinox, but decided against it, since he had made his choice. However, it was a ruse. Rhinox was still Tankor; except now instead of his strength being the focal point for inversion, it was his intelligence. He eventually discovered the Key to Vector Sigma, something Megatron had never been able to do. When he finally attempted to depose Megatron, however, a bombshell was dropped: Tankor was acting within the parameters of Megatron's programming the entire time. Megatron used both of Rhinox's defining characteristics -- his strength and intelligence -- for his own purposes at different times.
There are a couple of other things to briefly discuss. The Decepticon Air Commander Starscream possessed an immortal spark; he reappeared as a ghost twice in the original Transformers series and once in Beast Wars. Protoform X was a Maximal experiment designed to recreate Starscream's immortal spark. The result was Rampage. Megatron captured half of Rampage's spark and inflicted pain upon it when he wanted Rampage to do his bidding. It was highly effective in that regard. Eventually, though, Megatron used the other half of Rampage's semi-indestructible spark to create an evil clone of Dinobot. It seems as if these immortal sparks can be split and two independent personalities and life-forces are created. Then there's the opposite idea: the melding of two sparks. When Tigatron and Airrazor were abducted by the aliens called the Vok, their sparks were forged into one life-force and together they became the nearly godlike Tigerhawk. Of course, all good things must come to an end, and Tigerhawk was obliterated in due course.
So in conclusion...I have no conclusion. There's no easy way to neatly summarize the nature of the Transformer spark. I've laid out the facts available to me and I suppose now it's time for you to decide what to believe and what not to believe.
Did I leave something out? Have a different opinion? Do you just not like me? Do me a favor and /msg me and tell me why. Downvoting me without giving me an explanation is like reviewing a movie by saying "it sucks" and then not giving a reason. It doesn't help anybody improve anything and it certainly doesn't make me want to work any harder.