Hi Natasha, I’ll answer the second part of the question, because I don’t think there is a complex definition of the brain (there are lots of complex ideas, but they don’t amount to a definition – in a way this is part of the problem we face: can we make sense of all the different ideas about how the brain works, so that they can be summarized in a few, hopefully simple principles).
So, what are the main functions of the brain? This is a very interesting question, and other neuroscientists/psychologists would disagree with me. But I think we already know enough about the brain to see some big patterns which correspond roughly to different functions.
2) Planning and controlling responses to objects and events in our immediate environment.
3) Identifying and recognizing things, hearing, language and meaning.
4) Bodily sensation and movement.
5) Planning and deciding, controlling other parts of the brain to perform complex tasks.
6) Remembering events and places
7) Experiencing fear, remembering and avoiding danger.
8 ) Learning to do things that have worked well in the past.
Each of these points is associated with a particular part of the brain (1-5 are different parts of the wrinkley outer cortex; 6-8 are subcortical structures) which are more or less specialized for the things I’ve listed, but they interact with one another in complex behaviours. For example the parts of the brain involved in planning are linked to those involved with memory, language and meaning (and hearing) to allow speak and understand what others say.
There are some other parts of the brain I haven’t mentioned – these are the ones that are important to understanding thinking, feeling, remembering, seeing etc. (cognition).
The points listed above are quite vague, and for some parts of the brain (notably visual cortex) we have a much more detailed understanding of how they work. Some of the other parts of the brain are much less understood.
It’s not obvious (to me) that the brain can be divided up into different functions and separate chunks that perform those functions. It looks to me as if the different functions merge into one another as we move from one area of the brain to an nearby area.
Some of the things we might imagine are quite different (for example memory for everyday events and navigation) turn out to be linked quite closely in the brain, while others that seem essentially the same turn out to have multiple different systems, which can compete or work together to control our actions.