There are certain differences in the brain at different IQ levels, at least how I understand it.

At an IQ of 70-90, it's less that the brain is wired differently from an average human's and more that some part of the brain has a deficiency, or there is some messed connection, something along those lines. I'm not well-read here, but the rest of the answers should be somewhere in the comments section.

At an IQ of 100, you should know what happens, statistically. Someone can function normally in society, do basic things, work to do less basic things.

At an IQ of 110-130, you're a little above average. You may be a little shot emotionally, but you'll find something easier, maybe some things harder than an average person would.

At an IQ of 131-140, you're in the "highly gifted" category. You have better cognition, process things more comprehensively, you are all around a bit smarter. However, nobody's proficient in everything - some gifted people will be the worst people you've ever seen at mathematics, but be able to write the most eloquent thing you've ever read, beating out famous authors from throughout history. This is where the brain starts to diverge from that of a normal human - the human brain, for an average person, is doomed. It can evolve further, but only to a point. It gets stuck around the 130 IQ mark. After that, the brain beyond that point is formed in a different way from a regular person's brain, in the way of long connections and short connections. Where a normal person is relatively balanced in long/short, a genius will either be a one-trick-pony or a Renaissance man - someone with many short or many long connections, respectively. The "bottleneck" of our thought processing - where we perceive things - is where many things are cut off. Due to the nature of the mind, we notice everything, but don't take note of it all. In a gifted person's brain, they perceive far more from the same intake that a normal person gets. There's also the way they approach things - where a normal person will eliminate thoughts due to theories of them just not working, a gifted person will try them anyway just because they have nothing to lose. A gifted person can achieve more basically because they're willing to achieve more. It's a state of mind.

140-170 - I hate this IQ level. This is where long/short connections are at the extremes with mild hits to emotion. The brain will process things but have great problems with processing emotion. This is probably the closest to 70 IQ that you will have in the range, but they do everything either a lot better or a lot worse. Usually, this range will have such trouble with their emotions that it causes either mental disorders like depression (which practically removes the advantage a gifted person has by evidenced shrinking of the hippocampus and damage to other neurons in the brain), bipolar disorder, megalomania (or other extreme pride), or some crap like that, or they'll just shut themselves off and be perceptibly emotionless (until pushed pretty far). People tend to associate this level of giftedness with having a learning disability because they do extremely poorly in systems that encourage thinking "in a team (in a hivemind)." They're hard to tell apart from those with lower than average IQ because of what they're measured by. A normal person will pick up practical skills because that's what a normal person's life revolves around - that's why the most notable people in history have higher IQs - they're just abnormal, not exceptional.

A gifted person is on an entirely different spectrum, and because they're to the absolute extremes, if they're seen as bad at something, they'll eventually be seen as inferior rather than superior.

IQ 171-230 is where people have just horrible emotional problems and are incredibly good at things. The smartest theoretical physicists in history, Stephen Hawking and Albert Einstein, are both below this range. These guys could be good at everything or too good at one thing not to have sacrificed something else (like, I dunno, their soul) to get to where they are.

IQ is basically not how intelligent someone is, but how far they deviate from the norm. Towards 0 is dependent, the opposite way is also dependent.