Simply put indicators can be classified into 3 types, namely trendline indicators, oscillators and momentum indicator.
Trendline indicator has the main purpose to know the trend is happening with the existing range of periods (though trendline indicator can also be used to find other things like support and resistance points , etc.).
Oscillator indicator has distinctive characteristics that have a limited range of values, typically 0-100. RSI, Stochastic oscillator is an example of this type of indicator. Usually used to determine overbought and oversold point that will ultimately lead to an uptrend or downtrend.
Momentum indicators are used to determine how fast the acceleration of a trend that we can find out how long the trend will last.