Today I listened to Schubert's Piano Quintet in A major, D. 667. I can't find anywhere that says what the "D." stands for. Secondly why abbreviate something and make it impossible to find what the abbreviation stands for?
I think it is understandable that you did not realise what was going on if that was one of the first classical pieces you listened to. As you have probably gathered by now, the works of many classical composers have been catalogued, and the reference numbers are usually quoted to specify a piece. Looking at a decent article (e.g. Wikipedia) on the composer will probably tell you reasonably fast what applies to any given composer. In the case of Schubert, you see in the Wikipedia article (in the table of contents, if you have chosen to display it) the section Deutsch catalogue, which tells you what you want to know, namely that it refers to the catalogue by Otto Deutsch.
An alternative is opus (work) numbers, usually abbreviated to “Op.”, but these are usually given out during the composer’s lifetime, and may omit some pieces or otherwise be less reliable.