A seizure is an uncontrolled burst of activity in the brain. Different types of seizure produce different symptoms, from brief “blanking out”, to convulsions with loss of consciousness and muscle spasms.
At the onset of the seizure, and if you have experience in doing so, open the person's airways (mouth and nose) and assist them in breathing;
protect the person from injury;
turn the person onto one side with the head down to allow saliva drainage and prevent vomit inhalation;
stay with the person until they are fully recovered.
Read our article on what to do if someone has an epileptic fit for more info.