This is a common question. To the uninitiated, it can seem chaotic and disorganized. I will attempt to explain the basics of a jazz song as played by a jazz combo.

The skeleton of a jazz song is the "song". It can be an old song (e.g., "Misty", "The Girl from Ipanema", etc.) or less familiar melody composed by a jazz musician. This melody with form the "bookends" of the music, being played at the beginning and end of the song, excluding perhaps a short introduction and/or ending. The playing of the melody is often called, the "head." (It is called the "head" even when it is played at both the beginning and the end.) But ultimately, this typically takes up only a fraction of the time of the song, the majority (the middle section) being made up of the solos.

During the solos, the chord progression will function as the framework. This is the chord progression that ran under the melody and will now be repeated over and over under the solos. Each time through the chord progression is referred to as a "chorus" (This is a different meaning than is meant by pop/rock musicians.) Typically, the soloist will improvise a melody over a certain multiple of choruses. The improvised melody may be based to some extent on the original melody of the tune, but is often entirely original.

There is no set order of solos. But, combos tend to follow a default order of horns, chordal instruments, bass, then drums. While each horn plays its solo, the rhythm section (piano, vibes, guitar, bass, and/or drums) will "comp" (from "accompany" and/or "complement") the soloist, though typically only one of the chordal instruments will comp at a time. After the horns are done with their solos, the rhythm section will take theirs.

The chordal instruments will often comp for themselves, e.g., the piano player can play chords with his left hand while he improvises with his left. The guitar and vibes have a little bit more difficulty comping for themselves so one of the other chordal instruments may do it for them. The bass and drums are unique situations. The bass is a quiet instrument so often the drums will play softly or not at all and the comping will be soft, simple, or non-existant. (Please, yes, I know that things get very quiet during the bass solo, but don't start talking.)

A drum solo is even more different. A drum solo might be "trading." In "trading", typically another instrument will play for a certain number of measures (typically four, but could be some other fraction of the number of measures in the chorus) and then the drums will take his turn improvising for the same number of measures. During the drum solo trading, even though there is typically no one else playing, the chord progression is still going by. Another option is for the drums to solo by himself — with the chorus still going underneath, even though we are not hearing any chords. And lastly, sometimes the drummer will take a "free" solo, where the measures are not being counted, but the band will just take some cue to come back in.

This is of course a broad generalization. In reality, the song may start with a solo, the drums may be the first solo, maybe the trumpet and the guitar will trade, maybe the sax will play a chorus of his solo with no comping, etc. But the roadmap I have given is far and away the most common and is often the default into which we will fall if we have no plan.

I know I should say something like, "I imagine the ebulient joy of seeing my wife's hair cascade on her shoulders as we enjoy an Arubian sunset." But the answer I fear is much more prosaic - I am thinking about music. I am listening to what I am playing and thinking, "What comes next?" I am thinking, "There is a Db7#11 chord coming up in the next measure, I will try to slide this melody into an Eb major triad so that I can bring out the upper extensions better." I am trying to figure out how to develop the melodic figures I have used so far. In the back of my mind I am listeing to the comper to see if he is offering any melodic, harmonic, or rhythmic ideas. The bass player might offer harmonic ideas and the drums may offer and interesting rhythm. Overall, I am trying to give and interesting arc the solo, to take the listener on a journey.

I know this sounds like a lot. To some extent it is, but with practice it is manageable. Think about it, we often walk down the street, navigating the pedestrians, while talking on our cell phone, and then we dash across the street to the coffee shop, while uncounciously and effortlessly calculating the trajectories of half a dozen cars that are moving at various speeds and accelerations. This is of course a task of Herculean difficulty that we do without the slightest thought. How can we do this? Decades of practice. It is the same with musical improvisation.

It really depends. If you are in a club listening to a combo, the typical etiquette is to applaud for each soloist as they finish their solo. If the drums are trading, then you wait unitl the trading is over to applaud (which is usually when the final head begins.) You also applaud when the song is over.

If you are listening to a jazz combo play in stiuation where the music is not the main focal point, then the rules may change. If I am playing in a restaurant, then I usually don't expect the audience to applaud, as they are busy eating their meals and having their conversations — I would hate to impose on them by expecting applause. Sometimes there is something in between — maybe it is a jazz club that serves dinner. You just need to play it by ear. If I am playing in a combo playing for a wedding reception, then the soloists are not the focus and all I would expect is a bit of polite applause at the end of the song. But understand, this is my understanding. Unfortunately many jazz musicians have a bit of an ego and may expect applause in situations that I personally fell it is not required.