In Harry Potter and The Goblet of Fire, when Mr. Weasley and Mr. Diggory are leading a group to the Quidditch World Cup, Mr. Weasly explains:
I believe there’s a handy wood they’re using as the Apparition point. For those who don’t want to Apparate, or can’t, we use Portkeys. They’re objects that are used to transport wizards from one spot to another at a prearranged time. You can do large groups at a time if you need to.
However, this happens at the end of Chapter 31, The Third Task:
“On three, right?” said Harry. “One — two — three —”
He and Cedric both grasped a handle.
Instantly, Harry felt a jerk somewhere behind his navel. His feet had left the ground. He could not unclench the hand holding the Triwizard Cup; it was pulling him onward in a howl of wind and swirling color, Cedric at his side.
So, how come the Triwizard Cup didn't transport the holder at a prearranged time, but at the first touch?
The canonical answer might be "you just put a charm so it will transport the first holder instantaneously", but if it was so easy/simple, many wizards would have done it to get to the World Cup before they were supposed to (or after! if they didn't want to wait for so many days...).