Classical music titles: What does e.g. "No.2", "Opus 11", and the Roman numeral that precedes the title, mean?

The works of most classical composers are "catalogued" usually in the rough temporal order and assigned an opus number - starting with 1 for the first work. Sometimes these works involve a number of compositions, in which case they are numbered Opus 11, No.2, 3, etc.
Roman numerals are sometimes used, Hoboken, who catalogued Haydn's work liked this idea for catagorizing different types of compositions such as trios, quartets, symphonies, etc.

An example would have been helpful because your question is not very clear in exactly what you have seen.

Roman numerals were sometimes used to refer to the number of a work such as:

'VII Symphony' to mean 'seventh symphony', although this is quite archaic now and not common practice.

'Opus 11' would mean that it was labelled that way by the composer. 'Opus' simply means 'work' and so it would be the eleventh work in his own list of compositions. The 'No 2' part of your questuion is the confusing bit because it is unclear from your question where exactly it appears. If simply referring to a symphony (for example) eg 'Symphony No 2' it means it's the composers second symphony. However, if the context is 'Opus 11 No 2' then it means that the composer wrote and listed several works under one 'umbrella' opus No. This was common in the Classical era but much less so now. It was often used to collect works of a particular genre, eg Beethoven wrote 6 string quartets in his 'Opus 18' grouping. They are known as string quartets Op 18 No 1, Op 18 No 2 etc.

Hope that helps.

Here's an excellent overview on the

http://www.classical.net/music/composer/...

No. Means #
Opus means work