Time series databases (TSDB) are specifically designed to store time series data, most importantly by storing time/value pairs. The use of time as a key index is perhaps the most important factor distinguishing them from, for example, relational ones.
Because of the increased use of machine-generated data (e.g. IoT devices) they gained popularity relative to other database categories (by 180% over the last 24 months based on DB-Engines)
There are many different types of time series databases. As of October 2020 DB-Engines ranks the following ones as the top five: