Many people think that the astrological zodiacal signs Leo and Aquarius are the same as the "constellations". In a literal sense, a constellation is a collection of stars that seem to make up a figure if you would connect them by lines. Examples are "Ursa Major" and "The Big Dipper".
The 12 astrological signs are the most famous examples of constellations. The signs are the parts of the heaven through which the sun moves in the course of the year. Every part takes up 1/12 of the sky.
Planets and signs
Every astrological sign is accompanied by a certain planet. This planet, that belongs to your specific sign, can be in one of the other 12 astrological signs. The sign in which your planet stands is very important for interpreting your horoscope.
The reason is that every sign of the zodiac has its own energy pattern. If a planet stands in a specific sign, it will also show the characteristics of that specific sign.
If you are an Aries, for instance, then your planet would be Mars. If Mars is in the sign of Taurus, Mars will help you in a very different way to achieve your goals, then when Mars is in Gemini.