I don't know what you mean, so I'm giving meanings of both.
A greenhouse is a structure made from glass panes which is used to grow plants. It allows sunlight to enter but doesn't let it go back out. This helps keep the temperature warm so that the plants don't freeze and die.
Greenhouse effect is not used for anything. It is a natural phenomenon which occurs with the help of gases like carbon di - oxide. The ozone allows heat to enter inside the Earth's atmosphere so that the Earth remains warm, it acts like a blanket and prevents harmful radiations from entering the Earth. But lately, too much of pollution and overuse of CFLs has started weakening the ozone and letting harmful radiations inside. If this goes on then the Earth's temperature will rise so much that life will not be possible. This inflamation of the greenhouse effect is called "global warming"