Different titles and languages can be used to describe any one piece of music. For example, the same piano concerto by Mozart can appear as:
Concerto in A major for piano, K.488
Konzert A Dur, K.488 fur Klavier
Piano concerto in A Major, K.488
To allow for these variations in titles, music scores are catalogued using 'Uniform Titles'. Uniform Titles are collective titles that group similar forms of a composer's music together. For example, all of Mozart's symphonies are grouped by symphonies, concertos are grouped by concertos. This is why it's important to search the plural form (concertos, sonatas, etudes, suites) in your keyword search.
The Uniform Title for the above example would appear in the catalogue as:
Concertos, piano, orchestra, K.488, A major
Note the uniform title (above) is in English. The plural 'concertos' is used as Mozart composed more than one concerto. If you're unsure whether the composer wrote more than one particular genre form, place an asterisk at the end of the word (concerto*, sonata* etude*). This will search for all variations such as concerto, concertos.
Using Uniform Titles will help to improve your search results, so it's a good idea to be familiar with them!