A leading indicator gives a buy signal before the new trend or reversal occurs.
A lagging indicator gives a signal after the trend has started and basically informs you “hey buddy, pay attention, the trend has started, you’re missing the train.”
You’re probably thinking, “Ooooh, I’m going to get rich with leading indicators!” since you would be able to profit from a new trend right at the start. You’re right – you would “catch” the entire trend every single time, IF the leading indicator was correct every single time. But it’s not.
When you use leading indicators, you will experience a lot of fake-outs. Leading indicators are notorious for giving bogus signals which will “mislead” you.
The other option is to use lagging indicators, which aren’t as prone to bogus signals. Lagging indicators only give signals after the price change is clearly forming a trend. The downside is that you’d be a little late in entering a position. Often the biggest gains of a trend occur in the first few bars, so by using a lagging indicator you could potentially miss out on much of the profit.
Oscillators are leading indicators.
Momentum indicators are lagging indicators.