A time series is a sequence of data points that are recorded at specific times. Time series are often used in the fiance, trading, and housing industries. They are also used by scientists for predicting earthquakes, weather, and so on. Any data that has a certain pattern with regards to time (which could be days, weeks, months, or years) can be used to predict a future pattern (known as forecasting). In the trading industry, for example, it's useful to predict future stock prices. Please see the reference within the following note for more information on time series.
You can read more about the specialty at: https://onlinecourses.science.psu.edu/stat510/node/47/.
To illustrate a time series plot, let's use the FB.csv dataset. It contains fiance data related to FB, and was downloaded from Yahoo Finance. See the references at the end of the example for information on how this data was downloaded.