Bus Effects are used to process the sounds of many Voices simultaneously in real-time. They process the sound at the audio sample level so they can be very costly.
The Reverb is an example of effect which is often used.
Whenever you want to change the waveform data in real-time (without modifying the original waveform) you can use a effect plugin.
Delay, chorus, distortion and pitch shifter are some of the other commonly used effects.
Although effects are useful, using them has a non-negligible impact on both CPU load and memory usage.
It is recommended to reduce their usage on target platforms with modest performance.