These type of financial options are only exercisable on the expiry date of the option itself and not anytime before that.
They are, of course, less valuable than American style options, as they are more limited.
morgdx's example is seriously flawed. The figures quoted are hardly real world figures.
For buying at the money options, it is highly unlikely that at the money American style options would be selling for ten times the price of the same strike price European style options. The other thing is that the absolute price quoted is dodgy - $100k to buy options over 1 million units for one year at a strike price at the money for a period of 12 months is way too cheap ... though not as underpriced as the same European options mentioned later.
It's not that easy to make that much money in the real world.