As I understand it a holter monitor records all activity. I had one where you didn't have to press anything. I didn't feel ANY missed beats during the 24 hours but my gp said it recorded some (didn't say how many, just that they were nothing to worry about). Annoying when I have days when I feel hundreds of them, but happened to have the monitor on a 'good' day. I asked about this and he suggested that a serious arrythmia doesn't come and go and if it didn't show up in 24 hours it wasn't serious.
An event monitor usually just records when you press the button - for say 30 seconds. There are other ones however, which are continually recording and erasing data, and when you press a button it stops and records what happened just before and after you pressed. This explains different types of monitor:
http://www.nhlbi.nih.gov/health/heal...olt/types.html