There's always the fader on the mixer to control volume.But what is the difference between it and the compressor? I know you can add attack release etc on the compressor, but if Ι only want a sound to sound quieter whats the difference then?
A fader and a compressor are two different things.
compression is an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds thus reducing or compressing an audio signal's dynamic range. 1
In audio engineering, a fade is a gradual increase or decrease in the level of an audio signal. [...]
A fader is any device used for fading, especially when it is a knob or button that slides along a track or slot. 2
You might have confused them because the compressor might lower the volume of an audio input if it is set at a low limit, something that you can do with a fader, but it's not exactly the same.
To understand exactly what compression is, think it of a process that boosts the quiet signals and attenuates the louder ones, whereas the fader simply controls the volume of the input audio.
Here is a great article explaining many aspects of compression for beginners: