Instrumental. This is part of the song without any vocals. Yeah, okay. It's often an instrumental version of the verse or chorus, it may be an improvised variation on one of these, or it may be an entirely different tune and set of chords altogether. Sometimes it fits into a song where a vocal middle eight would otherwise go.

Breakdown/Break. This term has been high jacked from songs from the early 1900s when it was common to either to reduce the instrumentation or stop it altogether while a tap dancer would strut his stuff. The term 'break' is still sometimes used to indicate an instrumental section. 'Breakdown' is now most commonly used in dance music for the section where the percussion breaks down or is reduced, and it may be the dance equivalent of the middle eight.

Outro/Ending. Once upon a time, songs had definite endings but the mid 1950s heralded in the era of the fade-out and songwriters thought they would never have to write an ending again. However, fade-outs became such clichés to the extent that fade out meant cop out so songwriters started writing endings again. With that in mind, you can do as you wish, and considering that the endings of most songs get talked over or cut short by radio DJs and mixed over by club DJs, you have only your artistic integrity and your CD listeners to answer to. Some songs work extremely well with fade outs but listen to songs in your chosen genre to see how other writers approach endings. But whatever you do, avoid like the plague the three time tag ending.

Hook. The hook is not a song part as such; rather it's the term used to describe the part of the song that people remember and sing. It's what they buy the record for. It's usually the chorus although it need not be the entire chorus, but simply a two- or four-bar phrase. It could be an instrumental riff as in Whiter Shade of Pale or Smoke on the Water, or a processed vocal as in Cher's Believe.

All together now

Having described the parts of a song, let's see how they are commonly arranged. The most popular arrangement by far is simply verse-chorus and repeat. Here are two variations on the theme:
Intro
Verse 1
Chorus
Verse 2
Chorus
Chorus
Outro
Intro
Verse 1
Verse 2
Chorus
Verse 3
Middle eight
Chorus
Chorus
Outro

You get the picture. However, these are conventions rather than rules so you can adapt, change or ignore them as you see fit. But they have developed for a reason and that is simply to make the song as immediately appealing to the listener as possible.