When looking at the description of different signs, I came across the above question quite a few times. Many people complain that their zodiac sign description doesn't match their personality at all. For example, someone with a Libra sign would rather stay at home than socialize and hang out with a friend or two instead of an entire party (very un-Libra like). This infobarrel is designed to explain why this happens and what to expect.
The answer is that your sun sign (the sign that is coordinated to the month of your birth) has very little to do with your personality. I know this is not what most people are used to hearing but it is true. Your sun sign is based on exactly where the sun was at the time of your birth and basically shows everything to do with your ego, your focus, your sense of integrity etc. In basic terms, your sun sign shows who you are in public, who you are on the outside; your individuality.
Now with every sun sign also comes a moon sign. The moon travels through all the zodiac signs within 28 days, spending 54 hours in each sign. Your moon sign is based on exactly where the moon was at the time of your birth and shows more of an in-depth analysis on your personality. The moon shows your emotions, your inner feelings etc. In comparison, while your sun sign shows how you do things, your moon sign shows how you feel things. This is why when looking at relationship compatibility you should always look up the compatibility between you and your lover's moon signs since they show much more about your feelings and how you can interact with other people.
Even though looking at your moon sign is much more accurate than looking at your sun sign when it comes to the emotional part of your personality, to find out even more about your character you have to look at your entire natal chart. Your natal chart shows exactly where the zodiac signs are in each planet and since ever planet represents a part of your life, it's definitely worth looking into. For example, Venus represents love and whatever sign happens to be in Venus at the time of your birth will determine how you love. Mars represents your work ethic and everything to do with your career. Whatever sign happens to be in Mercury will show you how you think and process things, whether you think before you act, process things logically or based on emotions.
In conclusion, you sun sign doesn't show who you really are because it is only one very small aspect of your personality. In order to see the whole picture, you need to take a look at your entire natal chart.