In Astrology any heavenly body that is seen to move through the heavens, from the Earth's perspective is referred to as a planet, even the Sun and Moon. In ancient times the only visible planets were the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. The 3 outer planets - Uranus, Neptune and Pluto weren't discovered until after the invention of the telescope and are referred to as the 'modern' or even the 'generational' planets, as their orbits are so long that everyone born within a certain age group will all have these planets in the same Sign.
Where the planets are in your birth chart has a major influence on both who you are and how you approach life but also what you will go through at specific times in your life, depending on not only where the planets are but how they are interacting with each other and the other planets in your Chart. Each planet has a different energy and a different impact on us, but also for a different length of time. The Moon for example will visit all 12 areas of your Chart once a month and the Sun, Venus and Mercury will visit each area once a year.
However Mars will normally visit only once every other year, Jupiter once every 12 years and Saturn every 30 years. The outer planets will most probably never visit every area of your Chart in your lifetime. The longer it takes for a planet to return to your Sign the longer it will stay when it gets here and therefore the bigger the impact. The Moon for example will spend just 2.5 days in one Sign and therefore in one area of your Chart during each visit, while Neptune will spent up to 14 years.