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I confront this problem when I using data from Neurosky. The eight EEG powers are: delta (0.5 - 2.75Hz), theta (3.5 - 6.75Hz), low-alpha (7.5 - 9.25Hz), high-alpha (10 - 11.75Hz), low-beta (13 - 16.75Hz), high-beta (18 - 29.75Hz), low-gamma (31 - 39.75Hz), and mid-gamma (41 - 49.75Hz). I have found the meanning of some, for example, Delta reflects sleep and unconscious. However, the resource decribing the meaning of low-Alpha, high-Alpha, low-Gamma, mid-Gamma is too much little.
The definition of each frequency band is also variable. And, their roles can also be dependent on their brain location. We add some insights about this gamma in a recent paper. Give it a read: Article Cortical functional topography of high-frequency gamma activ...
Like João noted, the boundaries of these frequency bands are quite flexible and at the end of the day, they're going to be somewhat arbitrary. The power of these different frequency bands even differs from person to person which is why some researchers extract individual frequencies' peaks. The function of these different frequency bands is hotly debated as well and again, as João mentioned, these functions change depending on where they originate in the brain and even the context of the task in some cases. For example, alpha activity from the visual cortex can be thought of as inhibitory activity but when the same frequency band is recorded from the motor cortex, it's called mu and thought to represent motor preparation. As of now, there is no real consensus on the functional significance and precise boundaries of neural oscillations but that's why we continue to research them!
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