How do electrodes really work, and how to use them

Hi,

EEG is a mystery to me, especially when it comes to electrodes.
I just purchased an EEG-SMT device with one passive electrode (DPL) and 4
active electrodes.

>From what i understand, an active electrode differs from a passive one
because it has a built-in amplifier (right ?).
I still don't understand the concept of + and - electrodes, given that each
electrode is already, i think, a couple of electrodes (to measure a voltage
difference).

Apart from the theory (which i still don't understand, as you can see), i
have some practical questions regarding electrodes. Can someone give me a
link or a quick explanation of the best use of these electrodes ? I got the
DPL on earlobe thing (reference), but i don't really figure out how to
place the electrodes correctly : which type of gel do you need exactly ? Do
you need it with active electrodes also ?

I can measure a signal and make some math with it, but in the end i'm very
unsure about whether i am correctly monitoring the brain activity or just
random noise...

Thanks for your help

Alex

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