Is it just a monitor of the whole band sound coming from the mixing console? (same as the monitor speakers used for decades, placed in front of the musicians)

Is it just their own instrument?

Do they hear a click track for starting the songs? or perhaps a permanent click to keep a perfect tempo during the tracks?

Do they hear any comments from the sound engineer?

You mean earbuds/phones?   I occasionally used earplugs but that was to reduce noise damage to the ears.   As for what they hear I suppose it depends on the band, but I doubt many rock bands (do rock bands still exist?) use a click-track.   I'm guessing it's the monitor mix.

^ Cool

 

Personally I like a mix with the singer completely removed. . But that might just be because of our singer.

I remember last time I saw Transatlantic, Pete lost his monitors about 30min into the show, so he simply played based on feel with Portnoy, he did not move much away from Portnoy's kit area. Today not having the huge monitors on stage the band does not hear the mix well, I suspect on stage today it sounds like one big wall of muffled sound without in ear monitors.

Singers just want to hear themselves!

 

so true.

Singers are like baseball pitchers...........Prima Donna's!! 

Question... what is a monitor?

A monitor in this context is just a speaker - the term is typically used for live performance for a speaker pointed at a musician (usually a wedge on the ground in front of them), or as mentioned, wireless earbuds. It's mixed so the musician can hear (or "monitor") themselves well and any other musicians they need to hear, as opposed to being mixed so the audience gets a good balance of all the musicians.

Sometimes the term is used for speakers used in a music studio for recording/production purposes, generally higher quality speakers that reproduce the music as accurately as possible.

I'm sure it depends on the band, but nowadays more and more bands are using backing tracks, so they'd need a click to line up with it properly. In some bands I've played with, just the drummer would listen to the click and the band would follow him/her - although I've found that gets annoying in sections where the drums would ideally stop, but you get this perpetual hi-hat click to keep everyone else in time. Drummers tend to love click tracks, other musicians not so much. I know in Dream Theater, Portnoy used to have a silent cowbell that only the band could hear to count them in.

I don't know if the sound engineer really needs to make comments to a pro band - maybe they'd scold amateurs if they turn up their instruments too loud or do stupid things with the mic that cause feedback. Of course, the sound engineer would communicate during soundcheck!

Spot on........also on tape decks with multi-heads there is a Monitor button usually marked Tape or Source, where you can hear what you are recording to tape and hear any adjustments you might make like volume, balance, mix, bias so on. So if the tape is going L to R it first passes over the REC head then passes over a PLAY head, the delay is very minimal, but allows you to "monitor" the recorded sound, rather than wait till you are done and then playback the whole thing and hear the results.