Short story: There are many different types of seizures and reasons for them. Over 1/3 of children (and adults) with seizures do not know what causes their them.
Long Story: This is a very short explanation of a very complex subject. But for friends or family of epileptics it might clarify a few things.
Epilepsy is generic term that describes a large number of diagnosis given to anyone who has more than 2 seizures and has an EEG that confirms unusual brain activity. Some can have seizures and not be epileptic. Perhaps febrile seizures, where the body temperature is so high the body convulses, or seizures from a brain tumor pushing against the right nerve, or even a severe panic attack. Someone may have more than 2 seizures with no documented (EEG) unusual brain activity. This is a gray area as to whether the doctor diagnoses "epilepsy".
Regardless of the why, the how is easier to categorize. Tonic-clonic (grand-mal) are the typical type of seizure one thinks about when the word "seizure" is mentioned. Tonic is the tightening of muscles and clonic is repeated jerking. They can both be present or just one. Myoclonic are a series of jerks with space in-between. Some seizures, like absense, are look like the person is just staring off into space. If someone tells you they have seizures this could look differently for everyone. Many epileptics have a variety of seizure types.
Many of these seizures are "triggered" by certain outside influences. Lack of sleep is a big one. Flashing lights is another. Perhaps a certain food, a stressful situation, illness, fever, chemicals, or just a general overload of processing in multiple capacities.
EEG's (electroencephalogram) analyze brainwaves and report abnormalities. If someone has "epilepsy" their brain has unusual activity which may interfere with a host of functions and processes which are not easily categorized. These processes can be physical, intellectual, behavioral, sensory, anything and everything having to do with the brain. These unusual patterns may be happening all the time, not just during a seizure. For example, after seeing Ruby's EEG, her neurologist was surprised she was not having seizures more frequently, much more frequently. There are many kids who may have abnormal EEG's without seizures. According to many studies those with autism and ADHD have abnormal EEG's and types of autism can be categorized using an EEG.
Doctor's generally describe these abnormalities as "mis-wiring" and neurons "mis-firing" at a synapse, or junction along a neural path. Cannabis seemingly interacts with the neuron receptors and can help this transmission. Here is a GREAT article to explain this phenomenon.
The Epilepsy Foundation has attempted to re categorize seizures and epilepsy, but be warned it may confuse you further.