Amplify...
This effect increases or decreases the volume of a track or set of tracks. When you open the dialog, Audacity automatically calculates the maximum amount you could amplify the selected audio without causing clipping

Bass Boost...
This is a safe, smooth filter which can amplify the lower frequencies while leaving most of the other frequencies alone. It is most effective if you don't try to boost too much; 12 dB is usually just right.
Echo...

A simple delay line.
This effect repeats the audio you have selected again and again, softer each time. There is a fixed time delay between each repeat.

Echo
The Echo effect is very simple and is not intended to be used in place of a Reverb effect, which simulates the sound of a room, concert hall, stage, or other natural environment.

FFT Filter...
This is the most general type of filter. You define a curve that shows how much louder or quieter each frequency in the signal should be made.

Noise Removal...

This effect is ideal for removing constant background noise such as fans,tape noise, or hums. It will not work very well for removing talking or music in the background.

Removing noise is a two-step process. In the first step, you select a portion of your sound which contains all noise and no signal, in other words, select the part that's silent except for the noise.

Fade In
Applies a linear fade-in to the selected audio. For a logarithmic fade, use the envelope tool.

Fade Out
Applies a linear fade-out to the selected audio. For a logarithmic fade, use the envelope tool.