Saddle Stitching (staple binding)
Essentially an A3 document folded in the middle and stapled to create an A4 booklet. Most instrumental parts are are bound this way.
Saddle Stitching may sound like a strange name for a binding process where wire staples are put through sheets of paper, but in the printing industry the collated sheets are often draped over a saddle-like apparatus during the stitching/stapling process, hence the name Saddle Stitching.

Wire bound
Used for A3 scores, some A4 scores and orchestral parts, A4 Landscape music (e.g. Organ music and some piano music).
Documents are bound with wire binding elements, again allowing the music to lie flat on the stand. 
For A3 documents, heavy duty 300gsm card is used for the back cover, and the front has both a card and a clear PVC (laminate) cover as standard.

Comb bound
Documents are made into a book bound with a plastic comb, allowing it to lie flat on the stand.
A4 scores, larger orchestral parts, piano parts, and some music that is in landscape orientation are either bound this way or wire bound.
Card covers (160gsm front and 250gsm back) are provided as standard.