Technical analysis is the forecasting of market prices by means of analysis of data generated by the process of trading. Technical analysis relies on the assumption that markets discount everything except information generated by market action, ergo, all you need is data generated by market action.
Technical Analysis is like weather forecasting, technical analysis does not result in absolute predictions about the future. Instead, technical analysis can help investors anticipate what is "likely" to happen to prices over time. Technical analysis uses a wide variety of charts that show price over time.