Civil twilight, nautical twilight and astronomical twilight
Civil twilight is the time before sunrise (and after sunset) when the sun is still not more than 6 degrees below the horizon. At this time it is still possible to carry out most of the common activities without artificial lighting.
Nautical twilight is the time preceding or following civil twilight - the sun is between 6 and 12 degrees below the horizon. You can already see the brightest stars and sea horizon, that was previously used to navigate (that is how originated the name of this twilight).
Astronomical twilight is the time preceding or following nautical twilight - the sun is between 12 and 18 degrees below the horizon. At this time, the sky is still faintly illuminated by sunlight dispersed in the atmosphere, so you can not see the least bright stars.