Part of the brain gets overexcited - lots of neurons (nerve cells) suddenly fire at the same time. There are lots of kind of seizure, and different names for each kind.
If this happens in just a small part of the brain, it's a "focal" or "partial" seizure. These can be absences (petit mal or perhaps aura - maybe a slight confusion for 15sec) or simple partial seizures or "deja vu". People sometimes say that an aura happens before a seizure, but it's really an absence or simple partial seizure. Some focal seizures might be a 10sec weird feeling unnoticed by other people, or a horrible 45sec feeling with lots of simultaneous thoughts racing around uncontrollably, and some have goosebumps on the skin or familiar thoughts or smells. I have focal seizures nearly every day.
Sometimes the abnormal hyperactivity spreads like a vicious circle to take over large parts (or the whole of) the brain, so that the focal seizure becomes a "generalized" one. Some of these cause you to lose consciousness and fall and muscles to get stiff (tonic) for, say, 2 minutes. Some make the body shake (clonic) after the tonic phase - these are the "tonic-clonic" (Grand mal) seizures.
Youtube has lots of videos of seizures or about them. I find a good explanation, with video demonstration, to be
https://www.youtube.com/watch?v=Qu7Xaii02b0