Since my recent Scientist Appreciation and its content regarding electroencephalography (EEG), I have received several questions about brain waves and just what they actually are. I think there is an incredibly common misconception about what constitutes brain waves and what said waves mean. To really answer that question, though, I should start with a basic description of the neuron, specifically the cortical pyramidal cell.
The cortical pyramidal cell is the 'typical' neuron. I have put typical in quotations because I don't mean typical in the context of it being the most common (that would be the granule cells of the cerebellum, as I discussed in a post a while ago), but rather I mean it in the sense that it is the cell that most people think about when a person uses the term neuron (at least as far as my experience goes). For a very basic diagram of a neuron, you can go here.
As the diagram shows, there are three main parts to a neuron: dendritic tree, the soma (cell body), and the axon. The diagram also displays the myelin sheath and the terminal boutons (it calls them buttons), but those are only tangentially pertinent to the discussion of brain waves. The axon is a neuron's main method of sending a signal, via what is known as an action potential, to a target cell. The dendritic tree is where most action potentials are received by a neuron, although sometime signals are also received on the soma - this will be important later and discussed in further detail. The junction between an axon and a dendrite is known as a synapse. Neurons generate a resting potential across their cell membranes of approximately -70mV with respect to the surrounding interstitial fluid. For a neuron to fire an action potential it must be excited to below some threshold. Exciting a neuron is typically caused by synapses in the dendritic tree, where an incoming action potential triggers a brief influx of positive ions (usually Na+) that causes a small increase in the local intracellular voltage. This spike in voltage is known as an excitatory post-synaptic potential (EPSP). Similarly, it is possible to inhibit a neuron (hyperpolarizing it), usually through an influx of Cl- ions. This is known as an inhibitory post-synaptic potential (IPSP) and tends to occur on the soma.
While I kind of glossed over some of the details, I hope the basic idea is clear enough: a neuron can be triggered to fire an action potential if it is suitably excited through EPSPs occurring in its dendritic tree in enough quantity (either spatially - a whole bunch of disparate EPSPs being triggered at once - or temporally - a set of EPSPs being repeatedly triggered in quick succession such that ion levels build up to the point that an action potential is triggered). However, this neuronal firing can be hindered by IPSPs occurring on the soma. Now that the basic electrochemical computing mechanism of a neuron has been described, imaging a whole set of these pyramidal neurons arranged basically like a forest of trees. The dendrites are sticking up like foliage, the soma and axon are spatially distinct below like trunks. Due to the propensity for EPSPs to occur in the dendrites and IPSPs to occur on the soma, a small electric dipole is created through the predominant flow of positive ions toward the dendrites and negative ions toward the soma. Though a fairly small dipole, since most pyramidal neurons are arranged in parallel within an area of cortex, the small dipole generated by each can function additively to create a potential difference that is actually large enough to be detected by an EEG electrode affixed to a subject's scalp.
By recording potential difference between to distinct points on a person (a reference electrode and another electrode), a voltage can be determined, amplified, and output to either a computer (more modern) or a paper recording device (what they used to do). The reference electrode placement is actual a point of some contention, as typically a researcher wants a neutral electrode, but that is virtually impossible to find. Typical choices for reference electrode include the centre of the forehead just about the eyes, the back of the head, and the neck. This potential difference between the two electrodes tends to oscillate with varying neuronal activity. It is this oscillating activity that is referred to as a brain wave.
The amplitude and frequency of the oscillating potential difference can relate information about the general state of a subject's brain, as well as help to identify some specific pathological conditions. I do not know a lot about the clinical applications of EEG, but in terms of things like reading other peoples' thoughts and other such pseudo-scientific crap that it simply doesn't make sense. It is impossible to know where in the brain the sources of an EEG signal come from, so it cannot even be used to identify for certain the region of the brain doing cogitation, nevermind specific neurons.
That is a basic review of what brain waves are. I hope it all made sense, but it is hard sometimes to gauge what is basic knowledge and thus doesn't need inclusion, and what needs further explanation for someone who hasn't studied something for a while. I'm not feeling particularly eloquent at the moment, however, so I think I ought to end here for now.