Compressing in music is about reducing the dynamic, meaning, the gap between the highest and lowest volume in a specific track. The compression is applying (meaning, decrease the volume i.e.) when the track reach a certain level (in dB), called the treshold. The amount of compression is called the ratio and the speed with wich the compressor is coming back to his default level is called the release. There is tons of compressor available, because, each compressor has its own sound signature, color and behavior. By changing the dynamic, a compressor change the tone of the track it is applying to. There's a lot of discussion and disagreements on the question : hardware compression vs digital compression (mean, plugin comps) to which I would giving my opinion (mainly because I find it's never ending question). Here's a list of only hardware compressors. - [Tube-Tech CL2-A](https://www.thomann.de/fr/tubetech_cl2a.htm) - ~4100€. Legendary tube compressor. Warm compression, frequency response from 5hZ to 60kHz. - [Avalon VT-737SP](https://www.thomann.de/fr/avalon_vt737sp_roehren_direct_signal_pfad.htm) - ~3300€. Industry standard and very versatile compression. More a channel strip than a compressor, but include one. - [Empirical Labs EL8 X Distressor](https://www.thomann.de/fr/empirical_labs_el8_x_distressor.htm) - ~1850€. The swiss knife of compressors. Very versatile, used on a lot of different music genre. Can be paire to use in stereo. - [Drawner 1968 MK II](https://www.thomann.de/fr/drawmer_1968.htm) - ~1300€. Very versatile and transparent tube compressor. Double mono or stereo.