Your brain uses electrical pulses to send messages across it. These pulses are usually short, like binary in a computer, on or off.
A seizure is where this electrical pulse doesn't stop like it should, or gets sent over and over, an electrical cascade of oh fuck.
Anyone, given enough triggers or stress on the body, can seize. Epilepsy just means you have a lower 'threshold' for seizing than normal.
Imagine a landslide- if your cliffside is made of rock, an epileptic's is made of mud.
It takes a lot less for us to have a landslide than for you.

A shaky seizure like you see in movies is called a tonic-clonic. Tonic meaning the stiffening of muscles, which is why people fall, clonic meaning the shaking.
You get just tonic, just clonic. You also get atonic, where someone just goes limp, and myoclonic, which is a single muscle group jerking sharply.
A tonic-clonic is a generalised siezure- meaning it is over the whole brain, the entire brain is acting up. You are unconcious throughout.
(It bothers you so much more than it bothers me. I don't remember any of mine)
You do however get focal seizures- these are where only part of the brain are acting up. Some people are conscious for these, focal aware seizures.

A good chunk of people with epilepsy don't know why they have it. Another good chunk of people with epilepsy show no signs on an EEG.
An EEG is where a technician will come and stick electrodes onto your head (a LOT of electrodes) and they'll look at the electrical activity in your brain.
However, the brain is big and the scalp is only at the surface of the brain*. Deeper activity generally gets missed. (*citation needed)
If seizures aren't controlled, and surgery needs to be considered, an EEG can be implanted IN the head to look at these deeper seizures, but even that's not foolproof.

Something I'd like you to know? One, my medication and brain damage from seizures gives me memory issues. Also, please for the love of god, don't put a spoon in my mouth.