Difference between Relational database and time series database?
Time series describes the type of data you will be storing.  The use case that springs to mind is experimental or research data being collected at different time points.  You may have seen the term "Data Warehouse". That too is another type of data.  As it happens, you can store these data "types" in a relational database.  For time series data this may not be optimal.  There are a number of vendors in the space that cater directly to time series data.  See the wiki page referred to above.  Good luck.