Neurons

There is some pretty solid evidence that most of our thinking (whatever that is) is done by our neurons.
Each of has about 65 billion of them.
There are other brain cells called Glia, but they seem to have a much less central role.
The scientific community has computational models of neurons.
The simplest are called point models, and the simplest example of this is the integrate and fire neuron (from Lapicque 1907 or McCullouch and Pitts 1943).
A neuron collects (integrate) activation from other neurons. If it gets enough it fires. This sends activation to other neurons.
This requires connections. These connections are called synapses.
There are more complex point models, and compartmental models (e.g. Hodgkin-Huxley 1952) are even more sophisticated.
They're all models. You can have models of different types of neurons and even of different particular neurons. So you can have 65 billion different models to make up a brain.