Does an eeg show seizures or just the brain waves?

BOTH possible: A multi-channel eeg (electro-encephalo-gram) shows brain waves as they are happening. If someone 'just happened' to have a seizure while connected to the eeg machine then that would show up also (but that event is unlikely). Eeg's usually show other precursors to seizures (e.g. Spike/wave/slowing).

EEG: Eeg shows brain activity in wave form which helps to diagnose certain conditions such as seizures, metabolic encephalopathies, encephalitis etc.