Astrology, simply explained, is the study of the skies.
From time immemorial man watched the sky at night - before light pollution and light entertainment. One can safely assume it was early man's very first hobby. And when you watch the sky, you start noticing things; like night following day; like the sun and the moon rising from the same direction every day; like the moon waxing and waning; like the days getting shorter in winter and longer in summer.
Upon closer inspection of the night sky, you would notice stars and planets, which seem to follow some sort of pattern, and seem to effect life on Earth.
Why should it? Good question. It's not easy to get your head around the concept of a mass of gas, so many millions of miles away, having an effect on the way one behaves, feels, and the experiences one encounters. But if we accept the influence the Sun has on us (obviously warmth and light, indirectly mood and nourishment) and the way the moon touches us (moving the tides, effecting the menstrual cycle), then why shouldn't other planets work with us in the same way?
After all, it is based on observation. As is science.
So the planet which appears to influence communication, and the mind was named after Mercury, the quicksilver Roman messenger God. The celestial beauty that is Venus was named so as it appeared to touch on people's love and home life. The angry red planet which shone bright when war was afoot was named after battle God Mars. Jupiter, King of the Gods gave his name to the large star which seemed connected with expansion and judgement, and Saturn, the primal father, was attached to the slow planet which ruled gradual change and justice.
A person's date of birth, therefore, incorporates different personality traits, and the way those are expressed. The different planets, upon birth, would be placed in different zodiacal signs which would show how the person would behave, feel and think. The same is true for births of relationships, companies and countries.
A lot of trouble can be saved by starting a project on the right day, and a lot of information can be given by attention to astrological detail. We mustn't let it influence our lives, but use it to our own advantage.