Beginnings and Endings
Intro sections can be any length - 4 or 8 bars would be fairly typical.
Some tunes have two intros in that they might have a "pre-intro" bit followed by the intro proper.
Some tunes have no intro at all ( check out "Yellow Submarine" by The Beatles for example ).
A fairly common intro is to use the first four bars of the verse instrumentally.
I use outro to mean a bit at the end of a tune that is more or less the same piece of music as the intro.
A Coda is a separate piece of music that is added on at the end of a tune to function as an ending.
It can be any length - on the album version of "Layla" by Derek and the Dominoes the coda is three and a half minutes long - as long as the actual tune itself!
Rallentando or Rall. is a device frequently used to end a piece.
It is a deliberate slowing down of the last few bars to telegraph to the audience that the end is coming up.
It can be a very useful way of ending tunes that don't have an obvious cut off point.
Big endings and full stops
You all know the Big Ending much beloved by rock bands where the drummer flies round the kit and the guitarist either does a fast strummed chord or some fancy lead work.
The hard part here is to get the "full stop" at the end of this flurry synchronised.
A good drummer will have a cue in his bag of tricks to lead the other band members to hit the full stop at the same time.
If you use this type of ending a lot it is well worth spending time practising the exact details of your ending as there is nothing more likely to get the crowd applauding than a well worked ending.
Although these can sound good on recordings they are best avoided in live music - generally regarded as " couldn't be bothered to work out an ending " - on tunes that seem impossible to end try the rallentando technique.
Spend as much time rehearsing beginnings and endings as you would on the tune itself