Adding equalization is one of the most important aspects of mixing music as it can allow certain instruments to be heard better individually as well as in the overall mix. Too much bass on too many of the tracks can add a muddiness or boominess to the overall sound that can distract the listener. Most mix engineers are interested in cutting certain frequencies rather than boosting them. Snare drums, vocals and clean guitar need their low-mids cut in many cases while bass guitar and kick drums may need high-mids cut to take away the harshness of the sound.
Compression keeps the dynamics of instruments in check. If the bass or guitar gets too loud, a compressor can add a smoothness that would otherwise be lost. Compression is heavily used on bass guitars, electric guitars and kick drums where dynamics are not necessarily appropriate. Compression should not be obvious, as over-compressed instruments will lead to an unnatural pumping sound that may ruin the mix.
You can add multiple effects to each track during the mixing process. Reverb is useful on the snare drum as well as on the vocals to add depth to each, making the recording more natural sounding. Delay, flanger and distortion can be added to certain parts of the song to make those parts stand out or to express a particular feeling in the music. The mix engineer can add the effects during the mix or program them if using a computer-based mixing suite.
Noise gates are often used on all the tracks that contain pauses. The gate can open and close to lower the noise when the track is not able to be muted manually. Gating is often used on some of the drums, especially the snare as the other drums often can be heard on the snare track. The mix engineer can adjust the gate to make sure that it sounds natural. Too much gating can add a strange pumping sound to the overall finished project.