I had to explain this to someone the other day.
Venus is often known as the 'Morning Star' or the 'Evening Star' depending on what stage of its orbit it is in.
Why is it only visible in either early morning or early evening? Well, being closer to the sun than the Earth in it's orbit, it is obvious that the observer must at least be partially 'facing' the sun.
During full daylight, the sun is far to bright and obscures the observation just as it does any other heavenly body (moon excepted).
For most of the night, by definition, the observer is 'facing' away from the sun.
So at certain times in its orbit, (the best time is when Venus is 'furthest' away from the sun to an observer from Earth looking along the plane of its orbit), the planet can be seen either as soon as the light from the sun diminishes sufficiently, until such time as it disappears from view behind the rotaing Earth (or visa versa if in the part of venus' orbit where it is visible from earth in the morning).