While BW was the first to coin the term "Spark" which has since become a virtually standardised reference to Transformers' 'souls', the concept itself originated in G1. In the G1 cartoon it was referred to as the "laser core," and the comics used "life essence." The G1 comics showed that Transformers couldn't simply be constructed. Shockwave had to steal the Creation Matrix from Optimus Prime in order to give life to more Decepticons. The G1 cartoon was inconsistent (as usual ). At times it seemed easy enough to just build Transformers and they'd be alive, like the Dinobots and the brain-hurting contradicting origins of Megatron and the Constructicons... other times it was shown that Vector Sigma was the source of life for Transformers, and they used it to give life to the Aerialbots and Stunticons. But then at other times it seemed that their "brains" (personality components) were the most vital things for TF life, such as the Combaticons. But then Gears seemed to operate just fine with at least part of his personality component missing. Aaahh... where would the G1 cartoon be without its continuity inconsistencies!

"What did you do to Gears, you monster? You turned him... nice!"
- Ironhide, Changing Gears

But essentially the Spark is a Transformer's anima. It's what imbues him/her with life. A brain allows them to think, but not necessarily be alive. Teletraan-1, Zoom-Zoom, BM Vehicons etc. are capable of thoughts (of varying levels) but they aren't actually living entities. They're artificially intelligent, much like droids in Star Wars. A Jedi may risk his/her life to save another living being, but not to save a droid. There's an episode in the Clone Wars when Anakin insists on rescuing Artoo Detoo and Obiwan immediately cautions him about forming material attachments and advises against it. It's only agreed that Artoo needed to be rescued when Anakin confesses that he's never had Artoo's memory wiped, and therefore he carries secret Republic information that they couldn't allow to fall into enemy hands (so thus the missing was really to retrieve the data, rather than to rescue a droid)).

An example of a Cybertronian with a living Spark but with a very primitive mind would be Transmutate. Transmutate suffered significant mental and cognitive impairment because her logic circuits were scrambled beyond repair, giving her childlike immaturity and innocence. The IDW comics also show Sparks being "grown" out of the ground in certain places. These "embryonic" Sparks seem to lack sentience (let alone intelligence), but are portrayed as still being living Sparks.